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Home , Almon Brown Strowger, Automatic Electric, Charles Bourseul, Crossbar switch, Dial tone, Gardiner Greene Hubbard

The subscriber set consists of the following parts: 1. 2. Receiver 3. connections to the telephone system 4. Ringing circuitry 5. Dial network

The instrument, which contains the microphone and the receiver, is called handset. The handset is placed on the cradle when the telephone is not in use. In this position it opens the and disconnects the handset from the telephone system. An electromagnet, called the ringer is connected to the telephone line on the exchange side, so that a ring can be received from the exchange when it is called. The exchange determines that whether the telephone is idle or busy or initiating a call by monitoring the dc current. A simplified circuit and Block diagram of the telephone set is shown in the figure.

Circuit diagram of subscriber's telephone set Telephone Set

Microphone in is regarded as transmitter. It is a transducer, which converts sound energy into electrical energy. There are different types of but carbon granules transmitter is the most widely used in the handset of the modern telephony. We will discuss the carbon granule transmitter only. It is based on the principle that the resistance of carbon granules is inversely proportional to pressure. The constructional details of the carbon transmitter, is illustrated in the figure.

It is the property of carbon that its resistance varies with pressure. The carbon transmitter does not produce any e.m.f. but only change its resistance with the changing pressure.

Carbon granules are placed between two electrodes in an insulated chamber. One electrode is fixed to the back of the chamber while the other electrode is attached with the movable diaphragm. The two electrodes are connected with the battery. The transmitter offers an electrical resistance to the flow of current, which is the resistance of the carbon granules. When the diaphragm moves inward and outward, due to sound pressure, the pressure on the carbon granules also changes. Thus the resistance of the carbon granules also varies with the changing pressure and hence the current flow between the two electrodes also varies. A current variation, corresponding to the sound pressure. Telephone Set Receiver

The sound reproducer in telephony is called receiver. The receiver does the reverse function of a transmitter. It is a device, which converts electrical energy into sound energy.The constructional details of the telephone receiver, is shown in the figure.

It consists of a diaphragm, permanent magnet and windings. When the incoming current passes through the windings, is produced. The magnetic flux follows the magnetic path, which consists of the iron path of the permanent magnet, the pole pieces, diaphragm and the two air gaps between the diaphragm and the pole pieces. Thus a varying magnetic pull is produced, which causes the diaphragm to vibrate in accordance with the signal current received and hence produce the sound. The permanent magnet is used to polarize the receiver. The pull on the diaphragm depends upon the magnetic flux density in the air gaps between the diaphragm and the pole pieces. Thus maximum change will be produced when the current changes in the coil. Side Tone

When we speak in front of the telephone transmitter, .we hear our own voice in the receiver as a feedback. With the help of this feedback we are able to adjust the volume of our voice according to different situations. When the level of this feedback is high, we lower our voice and when it is low. we speak louder This feedback voice is called side tone. Side tone may be defined as the reproduction of sound in the receiver picked up from the associated transmitter. Or the amount of voice power coupled from the transmitter to the receiver of the same telephone.

Too much side tone and complete absence of it is undesirable. In the former case, the speaker will keep his level of volume of voice very low and hence will affect the output of the transmitter. In the latter case, the telephone will-appear dead to the subscriber and this is a very uncomfortable feeling.

The desirable amount of side tone is that which we have in our daily free air conversation with each other across the table. Antiside Tone Circuit

To control the level of the side tone to the desirable amount anti-side ton induction coil (A.S.T.I.C) is used in the subscriber's telephone set. The anti-side tone induction coil serves the following.

1. Control the level of the side tone to the desirable amount. 2. Ensures that no dc current flows through the receiver. 3. Matching between the impedances of the transmitter and receiver with that of the line. A simple circuit arrangement for the reduction of side tone is shown in the Figure.

The principle of the arrangement can be explained as follow.

Consider the figure (A) in this case the transmitter is transmitting.If

Zb = Zi & L1 = L2 then the transmitter current divides equally in L1and L2. The magnetic field produced by these two windings will be equal and opposite and hence cancel each other's effect, thus no emf is induced in L1 and the side tone is completely eliminated. Practically L1and L2 are not exactly equal and hence some emf is induced in L1, thus the receiver receives a portion of transmitter current. In figure (B) the receiving current passes through LI and L2. the magnetic field produced are in the same direction and reinforced each other's effect. An emf is induced in the receiver circuit and current flows through the receiver.

The anti side tone circuit is modified to make it more Practical and this modified circuit is shown in the figure. The tree windings L1, L2 and L3 are magnetically coupled. L1and L2 are not equal and hence an emf is induced in L3 from the transmitter. The value of R, across which the receiver is connected, is such that the voltage drop across it is equal and opposite to the emf induced in L3. This results in a much reduced side tone in the receiver. Magneto Bell

Bell is a means of signaling. When the calling party wants to call the called party. He must first give him a bell to draw his attention. For this purpose magneto bells used in the telephone set of each subscriber. The magneto bell works on AC supplied by the exchange. The constructional detail of the magneto bell is shown in the figure .It consists of a permanent magnet, two windings on the two legs of the iron yoke and an armature with a hammer. The static flux produced by the permanent magnet complete their path through the two parallel magnetic circuits. As the static field in each circuit is equal, thus the armature remains stationary. When alternating current flows through the windings, electromagnets are produced. The two fields interact with each other, supporting at one side and weakening at the other alternatively. The armature is attracted by the stronger magnetic field and the hammer strikes the gongs alternatively, thus produce a sound.

Basics Of Communication

What is communication?

Communication is the means of transferring a message from a source to destination in it’s original form at the most economical rate. The Telephone Instrument

 The Telephone is an instrument that converts sound to electrical pulses & vice versa.  It is designed to carry voice from one point to another

 It has a permissible frequency range of 300 to 3400 Hertz.

The Telephone Set

Function of the

Telephone Set

o Converts sound into electric signals and vice versa o The switch hook is a mechanism that originates and finalizes calls

KTS, PBX, Hosted PBX, IP , CTI, iPBX and WPBX, 2nd Edition Authors: Lawrence Harte, Robert Flood Number of Pages: 86 Number of Diagrams: 42

Select a Format:

This book provides an introduction to the different types of private telephone systems, how they operate and common call processing features they offer. Private telephone systems are communication equipment and software that are owned, leased or operated by the companies that use these systems.

Private telephone systems are converting from company unique (proprietary) circuit switched systems to industry standard packet data voice (IP Telephony) systems. You will learn the basics of IP Telephony voice over protocol (VoIP) and why it is so important to more.... Sample Diagrams

There are 42explanatory diagrams in this book

Analog and Digital Telephone Stations

This diagram shows the difference between standard analog telephone stations and more advanced PBX stations. This diagram shows that analog receive their power directly from the telephone line and digital PBX telephones require a control section that gets its power from the PBX system. Analog telephones also use in-band signaling to sense commands (e.g., ring signals) and to send commands (e.g., send dialed digits). Digital telephones use out-of-band signaling on separate communication lines to transfer their control information (e.g., calling number identification). WHAT IS DTMF?

When you press a button in the telephone set keypad, a connection is made that generates a resultant signal of two tones at the same time. These two tones are taken from a row frequency and a column frequency. The resultant frequency signal is called "Dual Tone Multiple Frequency". These tones are identical and unique.

A DTMF signal is the algebraic sum of two different audio frequencies, and can be expressed as follows:

f(t) = A0sin(2*П*fa*t) + B0sin(2*П*fb*t) + ...... ------>(1)

Where fa and fb are two different audio frequencies with A and B as their peak amplitudes and f as the resultant DTMF signal. fa belongs to the group and fb belongs to the high frequency group.

Each of the low and high frequency groups comprise four frequencies from the various keys present on the ; two different frequencies, one from the high frequency group and another from the low frequency group are used to produce a DTMF signal to represent the pressed key.

The amplitudes of the two sine waves should be such that

(0.7 < (A/B) < 0.9)V ------>(2)

The frequencies are chosen such that they are not the harmonics of each other. The frequencies associated with various keys on the keypad are shown in figure (A).

When you send these DTMF signals to the through cables, the servers in the telephone exchange identifies these signals and makes the connection to the person you are calling.

The row and column frequencies are given below:

Fig (A)

When you press the digit 5 in the keypad it generates a resultant tone signal which is made up of frequencies 770Hz and 1336Hz. Pressing digit 8 will produce the tone taken from tones 852Hz and 1336Hz. In both the cases, the column frequency 1336 Hz is the same. These signals are digital signals which are symmetrical with the sinusoidal wave.

A Typical frequency is shown in the figure below:

Figure (B)

Along with these DTMF generator in our telephone set provides a set of special purpose groups of tones, which is normally not used in our keypad. These tones are identified as 'A', 'B', 'C', 'D'. These frequencies have the same column frequency but uses row frequencies given in the table in figure (A). These tones are used for communication signaling.

The frequency table is as follows:

Figure (C)

Due to its accuracy and uniqueness, these DTMF signals are used in controlling systems using telephones. By using some DTMF generating IC’s (UM91214, UM91214, etc) we can generate DTMF tones without depending on the telephone set.

In a Decadic , also called Loop Disconnect Dialing, a Direct-Current Pulse Train, representing each Digit, is produced by interrupting a continous Signal according to a defined Ratio. Figure 1 shows a Decadic Pulse Telephone.

Figure 1: Decadic Pulse Telephone

A Decadic Pulse Telephone is equipped with a with a Finger Plate over it. The Rotary Dial is designed to send Electrical Pulses. Figure 2 shows the Circuit of a Pulse Telephone. The 3 spring contacts: the Impulsing Contact (ICT), the Bypass Switch 1 (BP1) and the Bypass Switch 2 (BP2) are mounted inside the Rotary Dial. Figure 2: The Telephone is Ready to make a Call when the Hook Switch is Closed. The Handset is Off the Hook.

Making a Call:

When the Customer lifts the Handset (Off Hook), the Hook Switch is closed and a DC Loop between the Telephone Exchange and the Customer is formed, Figure 1. Therefore, a Continous Current DC flows through the Loop.

 While Dialing a Number, for example, 5, the Dial is drawn round in the Clockwise Direction to the Finger Stop Position and released. The Finger Plate will then return to the Rest Position under the influence of a spring. Meanwhile, the BP2 in the Rotary Dial is closed to make a Short Circuit, thus you hear no disturbance in the Earphone during Dialing.  The ICT then generates the Dial Pulses by closing and opening itself to interrupt the DC Current loop. The number of interruptions is equivalent to the Dialed Digit. This type of Telephone generates two additional Pulses, which are eliminated by the BP1.

 Besides, there is also a mechanical device called Centrifugal Governor, mounted inside the Rotary Dial, it helps to maintain a uniform speed of rotation.

 After finishing Dialing, the Pulses are then decoded at the Telephone Exchange that make a connection to the Called Customer. The Voice Signals from the Customer will be transmitted to the Earphone through an Isolating . The Earphone is connected to the Secondary Winding of the Isolating Transformer. This protects the Earphone from being damaged by DC Current. The Transformer also provides an Electrical Isolation between the Telephone Exchange and the Ear. In addition, two anti-parallel Diodes protects the Ear from too much Noise. If the Voltage in the Secondary Winding is over a certain level, one of the Diodes starts to Conduct and makes a Short Circuit for the Earphone.

 While talking to the Microphone, the Sound Signal is transmitted to the Called Customer through the Telephone Exchange.

Receiving a Call: When the Handset is on the Cradle, the Telephone is said to be "On the Hook", or ready to receive a Call, Figure 3. The Hook Switch is opened and the path to the right part of the circuit is disconnected.

 Whenever there is an Incoming Call, An AC Ringing Signal from the Telephone Exchange is transmitted to the Telephone. The Ringing Signal is generally 10 mA AC Signal, with a Frequency between 20 and 25 Hz, that activates an Electromagnet which operates a small Hammer to strike the Bells.

Figure 3: The Telephone is Ready to receive a Call when the Hook Switch is Opened. The Handset is On the Hook.

The DTMF Telephone, as its name implied, is based on a concept known as Dual Tone Multi- Frequency (DTMF), Figure 1. It generates a combination of two Tones for each Dialed Digit, and sends the Digits to the Telephone Exchange by Hearable Tones instead of Electrical Pulses as in Decadic Pulse Telephone.

Figure 4: DTMF Telephone

The DTMF Telephone is equipped with a Pushbutton Dial, in which 10 Dialing Digits (0 through 9), the Star “ ” and the Pound “#” symbols are assigned to specific Pushbuttons. The Pushbuttons are arranged in an two-dimensional array with four Rows and three Columns, as shown in Figure 2. Each Row and Each Columns is assigned a Tone of a specific Frequency, the Columns having Tones of higher Frequencies and the Rows having Tones of lower Frequencies. When a Button is pushed, a Dual-Tone Signal is generated. This Signal is a combination of Two Tones of different Frequencies, one from the Lower Frequency Group and the other from the Upper Frequency Group, and it is the reason for calling it “Dual Tone Multi-Frequency”. In this way, 7 (4 + 3) Tones of different Frequencies are used to generate 12 (4 x 3) combinations. For example, pushing the Button “5”, the Tones of 770 Hz and 1336 Hz are transmitted together to the Telephone Exchange. This Signal is decoded by the Telephone Exchange in order to determine which Digit was Dialed.

Figure 5: The Pushbutton Dial and its corresponding Frequencies Pairs

Making a Call:

When the Customer lifts the Handset (Off Hook), the Hook Switch is closed and a Circuit connection between the Telephone Exchange and the Customer Telephone is formed.

Generation of Signaling Tones

 As has been said, the operation of any Pushbutton generates a Signal composed of Two Tones, which last as long as the Button is pushed. Figure 3 shows the Circuit of a DTMF Telephone. Figure 6: The Telephone is Ready to make a Call when the Hook Switch is Closed. The Handset is Off the Hook

 There are two Oscillation Circuits for generating Tones at different Frequencies. Each Circuit consists of a Three-winding Coil (A, A', A" and B, B', B") and a Capacitor (CA and CB). Windings A and B have a number of Spring Contacts, divided in to Group KA and Group KB. There are seven Cranks under the Pushbutton Dial, they are shown as the dotted lines in Figure 3, four of them corresponds to Rows and three to Columns. The operation of pushing a Button results in the actuation of a Horizontal Crank and a Vertical Crank. When a Crank is actuated, it will close the corresponding Spring Contact. The closure of one of the KA and one of the KB Contacts connects each Capacitor to one of the taps on the associated Winding A and B. In this way, the Oscillation Circuits corresponding to the Dialed Number are setup.  Then, the actuation of the Horizontal Cranks will also put a Common Switch K beside the Pushbutton Array in motion. The Common Switch K will operate a set of Contacts in sequential order as listed in Figure 3. The order and function of each Contact is stated in the following: 1. Attenuates the Dial Tones in the Earphone so the Customer hears the Dial Signal at a comfortable level. 2. Powers up the Transistor. 3. Disables the Microphone so that no other Noise is received from it to interfere the Dial Signal. 4. Initiates the Dual-Tone Signal from the Oscillation Circuits. This Signal is sustained by Feedback Amplification through the Transistor and the Transformer action between the Secondary (A', B') and Tertiary Windings (A", B") of each Coil.

 The whole Signal Generation Circuit is mounted on the back of the Pushbutton Panel, making the Pushbutton Dial a self-contained unit that can be substituted for the Rotary Dial in a Decadic Pulse Telephone. The other parts of a DTMF Telephone are similar to those of a Decadic Pulse Telephone, thus the process of Making and Receiving a Call is similar to that of a Pulse Telephone. At the Telephone Exchange, the Tones are decoded and a Connection is made to the Called Customer.

Receiving a Call:

When the Handset is on the Cradle, the Telephone is said to be "On the Hook", or ready to Receive a Call. That is, the Hook Switch in Figure 3 is opened and the path to the right part of the Circuit is disconnected.

Whenever there is an Incoming Call, An AC Ringing Signal from the Telephone Exchange is transmitted to the Telephone. The Ringing Signal is generally 10 mA AC Signal, with a Frequency between 20 and 25 Hz, that activates a pair of Electromagnet which operates a small Hammer to strike the Bells.

In 1876, made the first Telephone, called the Bell Telephone, Figure 1. Telephone comes from the Greek word “tele,” meaning “from afar”, and “phone”, meaning voice or voiced sound.

Figure 1: Bell Telephone The main parts of the Bell Telephone are: an Iron Diaphragm with attached Permanent Magnet and a Coil of Copper Wires, Figure 2.

Figure 2: Cross Section of the Bell Phone

By talking to the ’s Telephone, the voice of the Sender makes the Diaphragm vibrating. Since the Magnet is attached on the Diaphragm, the vibration of the Diaphragm also makes the Magnet vibrating in the Coil and a Current is induced. This Induced Current signal is then sent to the Receiver’s Telephone through the Copper Wires.

At the Receiver’s Telephone, the process is vice versa. The Telephone takes the Induced Current signal and translates it back into physical vibrations of the Diaphragm. The Sound is reproduced and can be heard. Telephone History

Early Telephone Development For more information on Leyden jars, including photographs and instructions on how to build them, go this page at the Static Generator site: http://www.alaska.net/~natnkell/leyden.htm

A static electricity web page is here: http://www.sciencemadesimple.com/static.html

In 1729 English chemist Stephen Gray transmitted electricity over a wire. He sent charges nearly 300 feet over brass wire and moistened thread. An electrostatic generator powered his experiments, one charge at a time. A few years later, Dutchman Pieter van Musschenbroek and German Ewald Georg von Kleist in 1746 independently developed the Leyden jar, a sort of battery or condenser for storing static electricity. Named for its Holland city of invention, the jar was a glass bottle lined inside and out with tin or lead. The glass sandwiched between the metal sheets stored electricity; a strong charge could be kept for a few days and transported. Over the years these jars were used in countless experiments, lectures, and demonstrations.

In 1753 an anonymous writer, possibly physician Charles Morrison, suggested in The Scot's Magazine that electricity might transmit messages. He thought up a scheme using separate wires to represent each letter. An electrostatic generator, he posited, could electrify each line in turn, attracting a bit of paper by static charge on the other end. By noting which paper letters were attracted one might spell out a message. Needing wires by the dozen, signals got transmitted a mile or two. People labored with telegraphs like this for many decades. Experiments continued slowly until 1800. Many inventors worked alone, misunderstood earlier discoveries, or spent time producing results already achieved. Poor equipment didn't help either.

Balky electrostatic generators produced static electricity by friction, often by spinning leather against glass. And while static electricity could make hair stand on end or throw sparks, it couldn't provide the energy to do truly useful things. Inventors and industry needed a reliable and continuous current.

In 1800 Alessandro Volta produced the first battery. A major development, Volta's battery provided sustained low powered electric current at high cost. Chemically based, as all batteries are, the battery improved quickly and became the electrical source for further experimenting. But while batteries got more reliable, they still couldn't produce the power needed to work machinery, light cities, or provide heat. And although batteries would work telegraph and telephone systems, and still do, transmitting speech required understanding two related elements, namely, electricity and magnetism.

In 1820 Danish physicist Christian Oersted discovered electromagnetism, the critical idea needed to develop electrical power and to communicate. In a famous experiment at his University of Copenhagen classroom, Oersted pushed a compass under a live electric wire. This caused its needle to turn from pointing north, as if acted on by a larger magnet. Oersted discovered that an electric current creates a magnetic field. But could a magnetic field create electricity? If so, a new source of power beckoned. And the principle of electromagnetism, if fully understood and applied, promised a new era of communication

For an excellent summary of Christian Oersted's life, visit: http://www.longman.co.uk/tt_secsci/resources/scimon/mar_01/oersted.htm

In 1821 Michael Faraday reversed Oersted's experiment and in so doing discovered induction. He got a weak current to flow in a wire revolving around a permanent magnet. In other words, a magnetic field caused or induced an electric current to flow in a nearby wire. In so doing, Faraday had built the world's first electric generator. Mechanical energy could now be converted to electrical energy. Is that clear? This is a very important point.

The simple act of moving ones' hand caused current to move. Mechanical energy into electrical energy. Although many years away, a turbine powered dynamo would let the power of flowing water or burning coal produce electricity. Got a river or a dam? The water spins the turbines which turns the generators which produce electricity. The more water you have the more generators you can add and the more electricity you can produce. Mechanical energy into electrical energy.

(By comparison, a motor turns electrical energy into mechanical energy. Thanks to A. Almoian for pointing out this key difference and to Neal Kling for another correction.)

Click here for a clear, large diagram on turning mechanical energy into electrical energy. And it's a good fair idea!

I also have a page on easy to do electrical experiments for kids Again, good science fair ideas. Faraday worked through different electrical problems in the next ten years, eventually publishing his results on induction in 1831. By that year many people were producing electrical dynamos. But electromagnetism still needed understanding. Someone had to show how to use it for communicating.

For more information on Michael Faraday, visit the ENC at: http://www.enc.org/features/calendar/unit/0,1819,196,00.shtm (external link)

In 1830 the great American scientist Professor Joseph Henry transmitted the first practical electrical signal. A short time before Henry had invented the first efficient electromagnet. He also concluded similar thoughts about induction before Faraday but he didn't publish them first. Henry's place in electrical history however, has always been secure, in particular for showing that electromagnetism could do more than create current or pick up heavy weights -- it could communicate.

In a stunning demonstration in his Albany Academy classroom, Henry created the forerunner of the telegraph. In the demonstration, Henry first built an electromagnet by winding an iron bar with several feet of wire. A pivot mounted steel bar sat next to the magnet. A bell, in turn, stood next to the bar. From the electromagnet Henry strung a mile of wire around the inside of the classroom. He completed the circuit by connecting the ends of the wires at a battery. Guess what happened? The steel bar swung toward the magnet, of course, striking the bell at the same time. Breaking the connection released the bar and it was free to strike again. And while Henry did not pursue electrical signaling, he did help someone who did. And that man was Samuel Finley Breese Morse.

For more information on Joseph Henry, visit the Joseph Henry Papers Project at: http://www.si.edu/archives/ihd/jhp/papers00.htm (external link) From the December, 1963 American Heritage magazine, "a sketch of Henry's primitive telegraph, a dozen years before Morse, reveals the essential components: an electromagnet activated by a distant battery, and a pivoted iron bar that moves to ring a bell." See the two books listed to the left for more information.

In 1837 invented the first workable telegraph, applied for its patent in 1838, and was finally granted it in 1848. Joseph Henry helped Morse build a telegraph or that allowed long distance operation. The telegraph later helped unite the country and eventually the world. Not a professional inventor, Morse was nevertheless captivated by electrical experiments. In 1832 he heard of Faraday's recently published work on , and was given an electromagnet at the same time to ponder over. An idea came to him and Morse quickly worked out details for his telegraph.

As depicted below, his system used a key (a switch) to make or break the electrical circuit, a battery to produce power, a single line joining one telegraph station to another and an electromagnetic receiver or sounder that upon being turned on and off, produced a clicking noise. He completed the package by devising the system of dots and dashes. A quick key tap broke the circuit momentarily, transmitting a short pulse to a distant sounder, interpreted by an operator as a dot. A more lengthy break produced a dash.

Telegraphy became big business as it replaced messengers, the Pony Express, clipper ships and every other slow paced means of communicating. The fact that service was limited to offices or large firms seemed hardly a problem. After all, communicating over long distances instantly was otherwise impossible. Yet as the telegraph was perfected, man's thoughts turned to speech over a wire. In 1854 wrote about transmitting speech electrically in a well circulated article. In that important paper, the Belgian-born French inventor and engineer described a flexible disk that would make and break an electrical connection to reproduce sound. Bourseul never built an instrument or pursued his ideas further.

For more information on Bourseul and early communications in general, vist this German site: http://www.fht-esslingen.de/telehistory/1870-.html (external link)

I have a page on easy to do electrical experiments for kids. And adults who want to understand the basics (internal link)

In 1861 Johann Phillip Reis completed the first non-working telephone. Tantalizingly close to reproducing speech, Reis's instrument conveyed certain sounds, poorly, but no more than that. A German physicist and school teacher, Reis's ingenuity was unquestioned. His transmitter and receiver used a cork, a knitting needle, a sausage skin, and a piece of platinum to transmit bits of and certain other sounds. But intelligible speech could not be reproduced. The problem was simple, minute, and at the same time monumental. His telephone relied on its transmitter's diaphragm making and breaking contact with the electrical circuit, just as Bourseul suggested, and just as the telegraph worked. This approach, however, was completely wrong.

Reproducing speech practically relies on the transmitter making continuous contact with the electrical circuit. A transmitter varies the electrical current depending on how much acoustic pressure it gets. Turning the current off and on like a telegraph cannot begin to duplicate speech since speech, once flowing, is a fluctuating wave of continuous character; it is not a collection of off and on again pulses. The Reis instrument, in fact, worked only when sounds were so soft that the contact connecting the transmitter to the circuit remained unbroken. Speech may have traveled first over a Reis telephone however, it would have done so accidentally and against every principle he thought would make it work. And although accidental discovery is the stuff of invention, Reis did not realize his mistake, did not understand the principle behind voice transmission, did not develop his instrument further, nor did he ever claim to have invented the telephone.

The definitive book in English on Reis is: Thompson, Silvanus P. Phillip Reis: Inventor of The Telephone. E.&F.N. Spon. London. 1883

For other views and explanations of the Reis instrument, visit Adventures in Cybersound: http://www.acmi.net.au/AIC/REIS_BIO.html (external link) In the early 1870s the world still did not have a working telephone. Inventors focused on telegraph improvements since these had a waiting market. A good, patentable idea might make an inventor millions. Developing a telephone, on the other hand, had no immediate market, if one at all. , Alexander Graham Bell, as well as many others, were instead trying to develop a telegraph, a device to send several messages over one wire at once. Such an instrument would greatly increase traffic without the telegraph company having to build more lines. As it turned out, for both men, the desire to invent one thing turned into a race to invent something altogether different. And that is truly the story of invention.

Alan J. Rogers' excellent introduction to electromagnetic waves, frequencies, and transmission. All applicable to telephony. Really well done. (19 pages, 164K in .pdf)

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Resources

[Britannica definition]" Systems: Telephone: THE TELEPHONE INSTRUMENT" Britannica Online. "In modern electret transmitters, developed in the 1970s, the carbon layer is replaced by a thin plastic sheet that has been given a conductive metallic coating on one side. The plastic separates that coating from another metal electrode and maintains an electric field between them. Vibrations caused by speech produce fluctuations in the electric field, which in turn produce small variations in voltage. The voltages are amplified for transmission over the telephone line."

[Accessed 11 February 1999] 9

"[Piezoelectric] crystals are used as transducers to convert mechanical or sound energy into electrical energy in such things as , , and in sound and vibration detection systems."

"Piezoelectricity was first observed in 1880 when Pierre and Jacques Curie put a weight on a quartz crystal and detected a proportional electric charge on its surface. A year later the converse effect was demonstrated -- that is when a voltage is applied to a crystal, a displacement occurs which is proportional to the voltage." "Reversing the polarity of the voltages reverses the direction of displacement. The term piezoelectricity is derived from the Greek word piezein meaning to press. Hence, a piezoelectric crystal is one capable of producing electricity when subjected to pressure."

An anonymous writer in the July, 1964 Lenkurt Demodulator

Analog and digital signals compared and contrasted

Analog transmission in telephone working. At the top of the illustration we depict direct current as a flat line. D.C. is the steady and continuous current your provides. The middle line shows what talking looks like. As in all things analog, it looks like a wave. The third line shows how talking varies that direct current. Your voice varies the telephone line's electrical resistance to represent speech. Click here for another diagram that complements this illustration.

Below is a simplified view of a . Current goes on and off. No wave thing. There was no chance the Reis telephone described above could transmit intelligible speech since it could not reproduce an analog wave. You can't do that making and breaking a circuit. A pulse in this case is not a wave! (internal link) It was not until the early 1960s that digital carrier techniques (internal link) simulated an analog wave with digital pulses. Even then this simulation was only possible by sampling the wave 8,000 times a second. (Producing CD quality sound means sampling an 44,000 times a second.) In these days all traffic in America between telephone switches is digital, but the majority of local loops are analog (internal link), still carrying your voice to the central office by varying the current.

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The Inventors: Gray and Bell

Elisha Gray was a hard working professional inventor with some success to his credit. Born in 1835 in Barnesville, Ohio, Gray was well educated for his time, having worked his way through three years at Oberlin College. His first telegraph related patent came in 1868. An expert electrician, he co-founded Gray and Barton, makers of telegraph equipment. The Western Union Telegraph Company, then funded by the Vanderbilts and J.P. Morgan, bought a one-third interest in Gray and Barton in 1872. They then changed its name to the Manufacturing Company, with Gray remaining an important person in the company. To Gray, transmitting speech was an interesting goal but not one of a lifetime.

Alexander Graham Bell, on the other hand, saw telephony as the driving force in his early life. He became consumed with inventing the telephone. Born in 1847 in Edinburgh, Scotland, Graham was raised in a family involved with music and the spoken word. His mother painted and played music. His father originated a system called that helped the deaf to speak. His grandfather was a lecturer and speech teacher. Bell's college courses included lectures on anatomy and physiology. His entire education and upbringing revolved around the mechanics of speech and sound. Many years after inventing the telephone Bell remarked, "I now realize that I should never have invented the telephone if I had been an electrician. What electrician would have been so foolish as to try any such thing? The advantage I had was that sound had been the study of my life -- the study of vibrations."

In 1870 Bell's father moved his family to Canada after losing two sons to tuberculosis. He hoped the Canadian climate would be healthier. In 1873 Bell became a vocal physiology professor at College. He taught the deaf the visual speech system during the day and at night he worked on what he called a harmonic or musical telegraph. Sending several messages at once over a single wire would let a telegraph company increase their sending capacity without having to install more poles and lines. An inventor who made such a device would realize a great economy for the telegraph company and a fortune for his or her self. Familiar with , Bell thought he could send several telegraph messages at once by varying their musical pitch. Sound odd? I'll give you a crude example, a piano analogy, since Watson said Bell played the piano well.

Imagine playing Morse code on the piano, striking dots and dashes in middle C. Then imagine the instrument wired to a distant piano. Striking middle C in one piano might cause middle C to sound in the other. Now, by playing Morse code on the A or C keys at the same time you might get the distant piano to duplicate your playing, sending two messages at once. Perhaps. Bell didn't experiment with pianos, of course, but with differently pitched magnetic springs. And instead of just sending two messages at once, Bell hoped to send thirty or forty. The harmonic telegraph proved simple to think about, yet maddeningly difficult to build. He labored over this device throughout the year and well into the spring of 1874.

Then, at a friend's suggestion, he worked that summer on a teaching aid for the deaf, a gruesome device called the phonoautograph, made out of a dead man's ear. Speaking into the device caused the ear's membrane to vibrate and in turn move a lever. The lever then wrote a wavelike pattern of the speech on smoked glass. Ugh. Many say Bell was fascinated by how the tiny membrane caused the much heavier lever to work. It might be possible, he speculated, to make a membrane work in telephony, by using it to vary an electric current in intensity with the spoken word. Such a current could then replicate speech with another membrane. Bell had discovered the principle of the telephone, the theory of variable resistance, as depicted below. [Brooks] But learning to apply that principle correctly would take him another two years. Bell continued harmonic telegraph work through the fall of 1874. He wasn't making much progress but his tinkering gathered attention. , a prominent Boston lawyer and the president of the Clarke School for The Deaf, became interested in Bell's experiments. He and George Sanders, a prosperous Salem businessman, both sensed Bell might make the harmonic telegraph work. They also knew Bell the man, since Bell tutored Hubbard's daughter and he was helping Sander's deaf five year old son learn to speak.

In October, 1874, Green went to Washington D.C. to conduct a patent search. Finding no invention similar to Bell's proposed harmonic telegraph, Hubbard and Sanders began funding Bell. All three later signed a formal agreement in February, 1875, giving Bell financial backing in return for equal shares from any patents Bell developed. The trio got along but they would have their problems. Sanders would court bankruptcy by investing over $100,000 before any return came to him. Hubbard, on the other hand, discouraged Bell's romance with his daughter until the harmonic telegraph was invented. Bell, in turn, would risk his funding by working so hard on the telephone and by getting engaged to Mabel without Hubbard's permission.

In the spring of 1875, Bell's experimenting picked up quickly with the help of a talented young machinist named Thomas A. Watson. Bell feverishly pursued the harmonic telegraph his backers wanted and the telephone which was now his real interest. Seeking advice, Bell went to Washington D.C. On March 1, 1875, Bell met with Joseph Henry, the great scientist and inventor, then Secretary of the . It was Henry, remember, who pioneered electromagnetism and helped Morse with the telegraph. Uninterested in Bell's telegraph work, Henry did say Bell's ideas on transmitting speech electrically represented "the germ of a great invention." He urged Bell to drop all other work and get on with developing the telephone. Bell said he feared he lacked the necessary electrical knowledge, to which the old man replied, "Get it!" [Grosvenor and Wesson] Bell quit pursuing the harmonic telegraph, at least in spirit, and began working full time on the telephone.

After lengthy experimenting in the spring of 1875, Bell told Watson "If I can get a mechanism which will make a current of electricity vary in its intensity as the air varies in density when a sound is passing through it, I can telegraph any sound, even the sound of speech." [Fagen] He communicated the same idea in a letter to Hubbard, who remained unimpressed and urged Bell to work harder on the telegraph. But having at last articulated the principle of variable resistance, Bell was getting much closer. On June 2, 1875, Bell and Watson were testing the harmonic telegraph when Bell heard a sound come through the receiver. Instead of transmitting a pulse, which it had refused to do in any case, the telegraph passed on the sound of Watson plucking a tuned spring, one of many set at different pitches. How could that be? Their telegraph, like all others, turned current on and off. But in this instance, a contact screw was set too tightly, allowing current to run continuously, the essential element needed to transmit speech. Bell realized what happened and had Watson build a telephone the next day based on this discovery. The Gallows telephone, so called for its distinctive frame, substituted a diaphragm for the spring. Yet it didn't work. A few odd sounds were transmitted, yet nothing more. No speech. Disheartened, tired, and running out of funds, Bell's experimenting slowed through the remainder of 1875.

During the winter of 1875 and 1876 Bell continued experimenting while writing a telephone patent application. Although he hadn't developed a successful telephone, he felt he could describe how it could be done. With his ideas and methods protected he could then focus on making it work. Fortunately for Bell and many others, the Patent Office in 1870 dropped its requirement that a working model accompany a patent application. On February 14, 1876, Bell's patent application was filed by his attorney. It came only hours before Elisha Gray filed his Notice of Invention for a telephone.

Mystery still surrounds Bell's application and what happened that day. In particular, the key point to Bell's application, the principle of variable resistance, was scrawled in a margin, almost as an afterthought. Some think Bell was told of Gray's Notice then allowed to change his application. That was never proved, despite some 600 lawsuits that would eventually challenge the patent. Finally, on March 10, 1876, one week after his patent was allowed, in Boston, Massachusetts, at his lab at 5 Exeter Place, Bell succeeded in transmitting speech. He was not yet 30. Bell used a liquid transmitter, something he hadn't outlined in his patent or even tried before, but something that was described in Gray's Notice. Bell's patent, U.S. Number 174,465, has been called the most valuable ever issued. If you have QuickTime or another way to view .tif files you can view the document at the Patent and Trademark site (external link). Search for it by the number. Each page of the six page document is about 230K. And yes, it is very hard to follow. Patents are meant to protect ideas, not necessarily to explain them . . .

The Watson-built telephone looked odd and acted strangely. Bellowing into the funnel caused a small disk or diaphragm at the bottom to move. This disk was, in turn, attached to a wire floating in an acid-filled metal cup. A wire attached to the cup in turn led to a distant receiver. As the wire moved up and down it changed the resistance within the liquid. This now varying current was then sent to the receiver, causing its membrane to vibrate and thereby produce sound. This telephone wasn't quite practical; it got speech across, but badly. Bell soon improved it by using an electromagnetic transmitter, a metal diaphragm and a permanent magnet. The telephone had been invented. Now it was time for it to evolve.

For the definitive answer on who invented the telephone (A hint, it was Bell), and a link to Edwin S. Grosvenor's authoritative, well researched, and clear thinking site defending Bell, click here. (internal link)

How the first telephone worked

Simplified diagram of Bell's liquid transmitter. The diaphragm vibrated with sound waves, causing a conducting rod to move up and down in a cup of acid water. Battery supplied power electrified the cup of acid. As the rod rose and fell it changed the circuit's resistance. This caused the line current to the receiver (not shown) to fluctuate, which in turn caused the membrane of the receiver to vibrate, producing sound.

This transmitter was quickly dropped in favor of voice powered or induced models. These transmitted speech on the weak electro-magnetic force that the transmitter and receiver's permanent magnets produced.

It was not until 1882, with the introduction of the Blake transmitter, that Bell telephones once again used line power. The so called local battery circuit used a battery supplied at the phone to power the line and take speech to the local switch. Voice powered phones did not go away completely, as some systems continued to be used for critical applications, those which may have been threatened by spark. In 1964 NASA used a voice powered system described as follows:

"A network of 24 channels with a total of more than 450 sound powered telephones, which derive their power solely from the human voice, provide the communications between the East Area central blockhouse (left) and the various test stands at NASA's George C. Marshall Space Flight Center here. . ." The complete article is here: http://americanhistory.si.edu/scienceservice/007016.htm (external link)

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Resources

Brooks, John. Telephone: The First Hundred Years. : Harper and Row, 1975: 41

Fagen, M.D., ed. A History of Engineering and Science in the . Volume 1 The Early Years, 1875 -1925. New York: Bell Telephone Laboratories, 1975, 6

Grosvenor, Edwin S. and Morgan Wesson. Alexander Graham Bell :The Life and Times of the Man Who Invented the Telephone. New York: Abrams, 1997: 55

Rhodes, Beginning of Telephony 4-5, 13-14 Bell develops the idea for the telephone.

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The Telephone Evolves

At this point telephone history becomes fragmented and hard to follow. Four different but related stories begin: (1) the further instrument and all its parts, (2) the history of the telephone business, (3) the history of telephone related technology and (4) the history of the telephone system. Due to limited space I can cover only some major North American events. Of these, the two most important developments were the invention of the vacuum tube and the transistor; today's telephone system could not have been built without them.

Progress came slowly after the original invention. Bell and Watson worked constantly on improving the telphone's range. They made their longest call to date on October 9, 1876. It was a distance of only two miles, but they were so overjoyed that later that night they celebrated, doing so much began dancing that their landlady threatened to throw them out. Watson later recalled "Bell . . . had a habit of celebrating by what he called a war dance and I had got so exposed at it that I could do it quite as well as he could." [Watson] The rest of 1876, though, was difficult for Bell and his backers.

Bell and Watson improved the telephone and made better models of it, but these changes weren't enough to turn the telephone from a curiosity into a needed appliance. Promoting and developing the telephone proved far harder than Hubbard, Sanders, or Bell expected. No switchboards existed yet, the telephones were indeed crude and transmission quality was poor. Many questioned why anyone needed a telephone. And despite Bell's patent, broadly covering the entire subject of transmitting speech electrically, many companies sprang up to sell telephones and telephone service. In addition, other people filed applications for telephones and transmitters after Bell's patent was issued. Most claimed Bell's patent couldn't produce a working telephone or that they had a prior claim. Litigation loomed. Fearing financial collapse, Hubbard and Sanders offered in the fall of 1876 to sell their telephone patent rights to Western Union for $100,000. Western Union refused.

(Special thanks to William Farkas of Ontario, Canada for his remarks and corrections)

In 1876 begins. Click here for a short but nice history (internal link)

On April 27, 1877 filed a patent application for an improved transmitter, a device that made the telephone practical. A major accomplishment, Edison's patent claim was declared in interference to a Notice of Invention for a transmitter filed just two weeks before by . This conflict was not resolved until 1886 however, Edison decided to produce the transmitter while the matter was disputed. Production began toward the end of 1877. To compete, Bell soon incorporated in their phones an improved transmitter invented by Francis Blake.

Blake's transmitter relied on the diaphragm modifying an existing electrical current, an outside power source. This was quite different than the original invention and its improvements. Bell's first telephone transmitter used the human voice to generate a weak electro-magnetic field, which then went to a distant receiver. Bell later installed larger, better magnets into his telephones but there was a limit to what power the human voice could provide, Myer indicating about 10 microwatts.

On July 9, 1877 Sanders, Hubbard, and Bell formed the first . Each assigned their rights under four basic patents to Hubbard's trusteeship. Against tough criticism, Hubbard decided to lease telephones and license franchises, instead of selling them. This had enormous consequences. Instead of making money quickly, dollars would flow in over months, years, and decades. Products were also affected, as a lease arrangement meant telephones needed to be of rental quality, with innovations introduced only when the equipment was virtually trouble free. It proved a wise enough decision to sustain the Bell System for over a hundred years.

In September, 1877 Western Union changed its mind about telephony. They saw it would work and they wanted in, especially after a subsidiary of theirs, the Gold and Stock Company, ripped out their telegraphs and started using Bell telephones. Rather than buying patent rights or licenses from the Bell, Western Union decided to buy patents from others and start their own telephone company. They were not alone. At least 1,730 telephone companies organized and operated in the 17 years Bell was supposed to have a monopoly.

Most competitors disappeared as soon as the Bell Company filed suit against them for patent infringement, but many remained. They either disagreed with Bell's right to the patent, ignored it altogether, or started a phone company because Bell's people would not provide service to their area. In any case, Western Union began entering agreements with Gray, Edison, and Amos E. Dolbear for their telephone inventions. In December, 1877 Western Union created the American Speaking Telephone Company. A tremendous selling point for their telephones was Edison's improved transmitter. Bell Telephone was deeply worried since they had installed only 3,000 phones by the end of 1877. Western Union, on the other hand, had 250,000 miles of telegraph wire strung over 100,000 miles of route. If not stopped they would have an enormous head start on making telephone service available across the country. Undaunted by the size of Western Union, then the world's largest telecom company, Bell's Boston lawyers sued them for patent infringement the next year.

On January, 28 1878 , the first commercial switchboard began operating in New Haven, Connecticut. It served 21 telephones on 8 lines consequently, many people were on a . On February 17, Western Union opened the first large city exchange in San Francisco. No longer limited to people on the same wire, folks could now talk to many others on different lines. The public switched was born. Other innovations marked 1878.

For a detailed history of telephone exchanges, particularly dial, please see R.B. Hill's excellent history: http://www.TelecomWriting.com/EarlyWork.html

On February 21, 1878, the world's first telephone directory came out, a single paper of only fifty names. George Williard Coy and a group of investors in the New Haven District Telephone Company at 219 Chapel Street produced it. It was followed quickly by the listing produced by the oddly named Boston Telephone Despatch Company. [First directory]

In 1878 President Rutherford B. Hayes administration installed the first telephone in the White House. [First tele] Mary Finch Hoyt reports that the first outgoing call went to Alexander Graham Bell himself, thirteen miles distant. Hayes first words instructed Bell to speak more slowly. [Hoyt]

In that year the Butterstamp telephone came into use. This telephone combined the receiver and transmitter into one handheld unit. You talked into one end, turned the instrument round and listened to the other end. People got confused with this clumsy arrangement, consequently, a telephone with a second transmitter and receiver unit was developed in the same year. You could use either one to talk or listen and you didn't have to turn them around. This wall set used a crank to signal the operator. The Butterstamp telephone.

For another great page on the earliest commercial telephones go here: http://atcaonline.com/phone/index.html (external link)

On August 1, 1878 Thomas Watson filed for a ringer patent. Similar to Henry's classroom doorbell, a hammer operated by an electromagnet struck two bells. Turning a crank on the calling telephone spun a magneto, producing an alternating or ringing current. Previously, people used a crude thumper to signal the called party, hoping someone would be around to hear it. The ringer was an immediate success. Bell himself became more optimistic about the telephone's future, prophetically writing in 1878 "I believe that in the future, wires will unite the head offices of the Telephone Company in different cities, and that a man in one part of the country may communicate by word of mouth with another in a distant place."

Subscribers, meanwhile, grew steadily but slowly. Sanders had invested $110,000 by early 1878 without any return. He located a group of New Englanders willing to invest but unwilling to do business outside their area. Needing the funding, the Bell Telephone Company reorganized in June, 1878, forming a new Bell Telephone Company as well as the New England Telephone Company, a forerunner of the strong regional Bell companies to come. 10,755 Bell phones were now in service. Reorganizing passed control to an executive committee, ending Hubbard's stewardship but not his overall vision. For Hubbard's last act was to hire a far seeing general manager named Theodore Vail. But the corporate shuffle wasn't over yet. In early 1879 the company reorganized once again, under pressure from patent suits and competition from other companies selling phones with Edison's superior transmitter. Capitalization was $850,000. William H. Forbes was elected to head the board of directors. He soon restructured it to embrace all Bell interests into a single company, the National Bell Company, incorporated on March 13, 1879. Growth was steady enough, however, that in late 1879 the first telephone numbers were used.

On November 10, 1879 Bell won its patent infringement suit against Western Union in the United States Supreme Court. In the resulting settlement, Western Union gave up its telephone patents and the 56,000 phones it managed, in return for 20% of Bell rentals for the 17 year life of Bell's patents. It also retained its telegraph business as before. This decision so enlarged National Bell that a new entity with a new name, American Bell Company, was created on February 20, 1880, capitalized with over seven million dollars. Bell now managed 133,000 telephones. As Chief Operating Officer, Theodore Vail began creating the Bell System, composed of regional companies offering local service, a long distance company providing toll service, and a manufacturing arm providing equipment. For the manufacturer he turned to a previous company rival. In 1880 Vail started buying Western Electric stock and took controlling interest on November, 1881. The takeover was consummated on February 26, 1882, with Western Electric giving up its remaining patent rights as well as agreeing to produce products exclusively for American Bell. It was not until 1885 that Vail would form his long distance telephone company. It was called AT&T.

On July 19, 1881 Bell was granted a patent for the metallic circuit, the concept of two wires connecting each telephone. Until that time a single iron wire connected telephone subscribers, just like a telegraph circuit. A conversation works over one wire since grounding each end provides a complete path for an electrical circuit. But houses, factories and the telegraph system were all grounding their electrical circuits using the same earth the telephone company employed. A huge amount of static and noise was consequently introduced by using a grounded circuit. A metallic circuit, on the other hand, used two wires to complete the electrical circuit, avoiding the ground altogether and thus providing a better sounding call.

The brilliant J.J. Carty introduced two wireservice commercially in October of that year on a circuit between Boston and Providence. It cut noise greatly over those forty five miles and heralded the beginning of long distance service. Still, it was not until 10 years later that Bell started converting grounded circuits to metallic ones

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Part A Before continuing let's look at Strowger's achievement. The automatic dial system, after all, changed telephony forever. (pronounced STRO-jer) was born in 1839 in Penfield, New York, a close suburb of Rochester. Like Bell, Strowger was not a professional inventor, but a man with a keen interest in things mechanical. Swihart says he went to an excellent New York State university, served in the Civil War from 1861 to 1865 (ending as a lieutenant), taught school in Kansas and Ohio afterwards, and wound up first in Topeka and then Kansas City as an undertaker in 1886. This unlikely profession of an inventor so inspired seems odd indeed, but the stories surrounding his motivation to invent the automatic switch are odder still.

Thanks to Joe Oster for supplying Strowger's birthplace

The many stories suggest, none of which I can confirm, that someone was stealing Almon Strowger's business. Telephone operators, perhaps in league with his competitors, were routing calls to other undertakers. These operators, supposedly, gave busy signals to customers calling Strowger or even disconnected their calls. Strowger thus invented a system to replace an operator from handling local calls. In the distillation of these many stories, Stephan Lesher relates a story from Almon's time in Topeka:

"In his book, Good Connections, telephone historian Dave Park writes that Strowger grew darkly suspicious when a close friend in Topeka died and the man's family delivered the body to a rival mortician. Strowger contended that an operator at the new telephone exchange had intentionally directed the call to a competitor -- an allegation that gave rise to tales that the operator was either married to, or the daughter of, a competing undertaker."

Good connections : A Century of Service by the Men & Women of by David G. Park (Long out of print, but try htttp://www,abe.com)

Whatever the circumstances, we do know that anti-Bell System sentiment ran high at this time, that good telephone inventions commanded ready money, and that Strowger did have numerous problems with his local telephone company. Strowger was a regular complainer and one complaint stands out.

Swihart describes how Southwestern Bell personnel were called out to once again visit Strowger's business, to fix a dead line. The cause turned out to be a hanging sign which flapped in the breeze against exposed telephone contacts. This shorted the line. Once the sign was removed the line worked again. It may be supposed that this sort of problem was beyond a customer's ability to diagnose, that Strowger had a legitimate complaint. But on this occasion Southwestern Bell's assistant general manager, a one Herman Ritterhoff, was along with the repair crew. Strowger invited the man inside and showed him a model for an automatic switch. So Strowger was working on the problem for quite some time and was no novice to telephone theory.

Brooks says that, in fact, Strowger knew technology so well that he built his patent on Bell system inventions. It must be pointed out, however, that every inventor draws ideas and inspiration from previously done work. Brooks says specifically that the Connolly-McTighe patent (Patent number 222, 458, dated December 9, 1879) helped Strowger, a failed dial switchboard, as well as an early automatic switch developed by Erza Gilliland. But Strowger did not build the instrument since he did not have the mechanical skills. A rather clueless jeweler was employed instead to build the first model, and much time was wasted with this man, getting him to follow instructions.

As with Bell, Strowger filed his patent without having perfected a working invention. Yet he described the switch in sufficient detail and with enough novel points for it to be granted Patent number 447,918, on March 10, 1891. And in a further parallel with Bell, Almon Strowger lost interest in the device once he got it built. It fell upon his brother, Walter S. Strowger, to carry development and promotion further, along with a great man, Joseph Harris, who also helped with promotion and investment money. Without Harris, soon to be the organizer and guiding force behind , dial service may have taken decades longer for the Bell System to recognize and develop. Competition by A.E. forced the Bell System to play switching catchup, something they really only accomplished in the 1940s with the introduction of crossbar.

Need something technical on Strowger's work? I've put R.B. Hill's switching history article on line here: http://www.TelecomWriting.com/Switching/EarlyYears.html The citation to that article is here.For more on common battery and the last manual switchboard to be retired in America, click here

In 1897 Milo Gifford Kellogg founded the Kellogg Switchboard and Supply Company near . Kellogg was a "graduate engineer and accomplished circuit designer"[Pleasance], who began his career in 1870 with Gray and Barton, equipment manufacturers for Western Electric. There he developed Western Electric's best telephone switchboards: a standard model and a multiple switchboard. Both were invented in 1879 and patented in 1881 and 1884, respectively. He retired from Western Electric in 1885, "and began making and patenting a series of telephone inventions of his own, which work extended over a period of 12 years and which culminated in the issue of 125 patents to him on October 17, 1897, besides which over 25 had previously been issued to him."[Telephony] He was also quite political, successfully winning suits against Bell and delaying other Bell actions to his benefit. Telecom History called him "probably the man in the American independent telephone business who first placed himself in opposition to the Bell Company."[Telephony]

His major accomplishment was the so called divided-multiple switchboard, of which two were built. One was sold to the Cuyahoga Telephone Company of Cleveland, Ohio and the other to the Kinloch Telephone company of Saint Louis. The Cleveland installation boasted 9,600 lines, with an ultimate capacity of 24,000! Such large switchboards were needed to handle increasing demand. The Kellog boards were much larger than Bell equipment, mostly designed by Charles Scribner. Saint Louis and Detroit independents started switching to Kellog boards, "threaten[ing] Bell's profitable urban markets."[Grosvenor] Under such pressure and once again running out of money, Bell regrouped.

In 1899 American Bell Telephone Company reorganized yet once again. In a major change, American Bell Telephone Company conveyed all assets, with the exception of AT&T stock, to the New York state charted American Telephone and Telegraph Company. It was figured that New York had less restrictive corporate laws than Massachusetts. The American Bell Telephone Company name passed into history.

In 1900 loading coils came into use. Patented by Physics Professor Michael I. Pupin, loading coils helped improve long distance transmission. Spaced every three to six thousand feet, cable circuits were extended three to four times their previous length. Essentially a small electro- magnet, a or inductance coil strengthens the by decreasing attenuation, the normal loss of signal strength over distance. Wired into the transmission line, these electromagnetic loading coils keep signal strength up as easily as an electromagnet pulls a weight off the ground. But coils must be the right size and carefully spaced to avoid and other transmission problems.

Pupin's patent is U.S. number 652,230 which you can view at the United States Patent Office: http://www.uspto.gov (external link) His patent in 1900 caused almost as much controversy as Bell's telephone patents. As the crucial invention for extending long distance circuits it was an extremely valuable patent and hence contested by groups like AT&T which eventually bought the rights. It also served as an incentive for the Bell System to found . As Wasserman put it, AT&T had been "played to a virtual tie with a lone inventor working in an academic setting. . . This point was not ignored by management."

The definitive book on loading coil history and early long distance working is Neil Wasserman's book, From Invention to Innovation: Long Distance Telephone Transmission at The Turn of the Century. John Hopkins/AT&T Series in Telephone History. 1985.

Details from the patent. Click to enlarge

In 1901 the Automatic Electric Company was formed from Almon Strowger's original company. The only maker of dial telephone equipment at the time, Automatic Electric grew quickly. The Bell System's Western Electric would not sell equipment to the independents, consequently, A.E. and then makers like Kellog and Stromberg-Carlson found ready acceptance. Desperate to fight off the rising independent tide, the Bell System concocted a wild and devious plan. AT&T's president Fredrick Fish approved a secret plan to buy out the Kellog Switchboard and Supply Company and put it under Bell control. Kellog would continue selling their major switchboards to the independents for a year. At that time the Bell System would file a patent suit against Kellog, which they would intentionally loose. This would force the independents to rip out their newly installed switchboards, crushing the largest independents. The plan was discovered, aborted, and further scandalized AT&T.[Grosvenor2]

By 1903 independent telephones numbered 2,000,000 while Bell managed 1,278,000. Bell's reputation for high prices and poor service continued. As bankers got hold of the company, the Bell System faltered.

In 1907 Theodore Vail returned to the AT&T as president, pressured by none other than J.P. Morgan himself, who had gained financial control of the Bell System. A true robber baron, Morgan thought he could turn the Bell System into America's only telephone company. To that end he bought independents by the dozen, adding them to Bell's existing regional telephone companies. The chart shows how AT&T management finally organized the regional holding companies in 1911, a structure that held up over the next seventy years. But Morgan wasn't finished yet. He also worked on buying all of Western Union, acquiring 30% of its stock in 1909, culminating that action by installing Vail as its president. For his part, Vail thought telephone service was a natural monopoly, much as gas or electric service. But he also knew times were changing and that the present system couldn't continue.

In January 1913 the Justice Department informed the Bell System that the company was close to violating the Sherman Antitrust Act. Vail knew things were going badly with the government, especially since the Interstate Commerce Commission had been looking into AT&T acquisitions since 1910. J.P. Morgan died in March, 1913; Vail lost a good ally and the strongest Bell system monopoly advocate. In a radical but visionary move, Vail cut his losses with a bold plan. On December 19, 1913, AT&T agreed to rid itself of Western Union stock, buy no more independent telephone companies without government approval and to finally connect the independents with AT&T's long distance lines. Rather than let the government remake the Bell System, Vail did the job himself.

Known as the Kingsbury agreement for the AT&T vice president who wrote the historic letter of agreement to the Justice Department, Vail ended any plans for a complete telecommunications monopoly. But with the independents paying a fee for each long distance call placed on its network, and with the threat of governmental control eased, the Bell System grew to be a de facto monopoly within the areas it controlled, accomplishing by craft what force could not do. Interestingly, although the Bell System would service eighty three percent of American telephones, it never controlled more than thirty percent of the United States geographical area. To this day, 1,435 independent telephone companies still exist, often serving rural areas the Bell System ignored. Vail's restructuring was so successful it lasted until modern times. In 1976, on the hundredth anniversary of the Bell System, AT&T stood as the richest company on earth.

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Resources:

Grosvenor, Edwin S. and Morgan Wesson. Alexander Graham Bell: The Life and Times of the Man Who Invented the Telephone. Harry N. Abrams, New York (1997) 167 Excellent. Grosvenor2. ibid, 167

Brooks, John. Telephone: The First Hundred Years. Harper & Row, New York. 1975, 1976: 100

Hill, R.B. "The Early Years of the Strowger System" The Bell Laboratories Record March, 1953: 95

Swihart, Stanley. "The First Automatic Telephone Systems" Telecom History: The Journal of The Telephone History Institute No. 2. Spring, 1995: 3

Pleasance, Charles A., "The Divided Multiple Switchboard" Telecom History: The Journal of The Telephone History Institute 1 (1994) 102

"Well-Known Heads of Well-Known Houses", Telephony (July, 1901) As reprinted in Telecom History: The Journal of The Telephone History Institute 1 (1994) 93 ibid 93

Added note

Q. I remember hearing once about how with point-to-point connections, required before switchboard exchanges evolved, could "darken the skies" in urban areas -- and I remember seeing a photo of just that -- a thicket of lines criss-crossing between offices in some downtown area. I think it might have been the loop in Chicago. Do you have an info on this -- specifically I would love to find that photo or a similar one.

A. They indeed could darken the skies. A welter of open wire like that was not only unsightly but could be wrecked by a wind or ice storm. The photograph I am linking to is of but the site was common in most large cities. It's a great before and after illustration: http://www.uh.edu/engines/nycandwires.jpg (external link)

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Part B

At this point we need to look back a few years. In 1906 invented the three element electron tube. Its properties led the way to national phone service. Long distance service was previously limited to 1,500 miles or so. Loading coils and larger, thicker cables helped transmission to a point but no further. There was still too much loss in a telephone line for a voice signal to reach across the country. Transcontinental phone traffic wasn't possible, consequently, so a national network was beyond reach. Something else was needed. In 1907 Theodore Vail instructed AT&T's research staff to build an electronic amplifier based on their own findings and De Forest's pioneering work. They made some progress but not as much as De Forest did on his own. A nice De Forest biography is at: http://www.acmi.net.au/AIC/DE_FOREST_BIO.html (external link) The site also includes the photograph below.

The most popular book on De Forest is Empire of the Air : The Men Who Made Radio by Tom Lewis. Try searching for it with the Powells.com search engine at the bottom of this page.

AT&T eventually bought his patent rights to use the tube in their telephone amplifier. Only after this and a year of inspecting De Forest's equipment did the Bell Telephone Laboratory make the triode work for telephony. Those years of research were worth it. Electron tube based amplifiers would make possible radiotelephony, transmission, radar, , and hundreds of other technologies. Telephone could now span the country, enabling a nationwide telephone system, fulfilling Alexander Graham Bell's 1878 vision.

Recalling those years in an important interview with the IEEE, Lloyd Espenschied recounts "In May [1907], several of us had gone to a lecture that Lee De Forest had given on at the Brooklyn Institute of Arts and Sciences. In this lecture, he passed around a queer little tube to all the audience. It was the first three-element tube to be shown in public, I found out afterwards. He passed this around and everybody looked at it and said, "So what!" Even De Forest said that he didn't know what it was all about. He looked on it as a detector. [an early device to pick up radio waves, ed.] Actually it was an evolution of the Fleming valve, but he would never give credit to anyone." Later in the interview, Espenschied gives an opinion of De Forest shared by many at the time, "No, he was no engineer. He was just a playboy all his life. He's just plain lucky that he stumbled into the three- element device. Just plain lucky. But that was handed to him for persevering; he kept at it, grabbing and grabbing at all the patent applications without knowing what he was doing."

For more quotes like the above and a great oral history of early electronic and vacuum tube experimenting: http://www.ieee.org/organizations/history_center/oral_histories/transcripts/espenschied11.html (external link)

Luck or not, De Forest was first to build and then exploit the the three element tube. It later enabled the vacuum tube repeater which ushered in telephony's age. A triode is sometimes called a thermionic valve. Thermions are electrons derived from a heated source. A valve describes the tube's properties: current flows in one direction but not the other. Think of a faucet, a type of control valve, letting water go in only one direction. This controlled flow of electrons, not just electricity itself, marks the end of the electrical age and the beginning of the electronic age.

Go here for more on de Forest and how the triode works (internal link)

For more comments, read Ray Strackbein's comments below

Armstrong later developed the regenerative circuit which fed back the input signal into the circuit over and over again. In electronic books of the era many called him "Feedback Armstrong." His circuit amplified the signal far more than original designs, allowing great wireless or wireline transmission signal strength. The feedback circuit could also be overdriven, fed back so many times that supplying a small current would develop an extremely high frequency. The circuit would thus resonate at the frequency of a , letting the triode receive or detect signals, not just transmit them. DeForest later claimed to have invented regeneration; this was a lie. DeForest invented the three element tube by trial and error; he did not even understand how it worked until five years later when Edwin Armstrong explained it.

More on this regarding radio is here (internal link)

As evidence of the triode's success, on January 25, 1915 the first transcontinental telephone line opened between New York City and San Francisco. The previous long distance limit was New York to Denver, and only then with some shouting. Two metallic circuits made up the line; it used 2,500 tons of hard-drawn copper wire, 130,000 poles and countless loading coils. Three vacuum tube repeaters along the way boosted the signal. It was the world's longest telephone line. In a grand ceremony, 68 year old Alexander Graham Bell in New York City made the ceremonial first call to his old friend Thomas Watson in San Francisco. In an insult to Lee de Forest, the inventor was not invited to participate. This insult was carried over to the 1915 World's Fair in San Francisco, in which AT&T's theater exhibit heralded coast to coast telephone service without mentioning the man who made it possible. [Morgan]

Professor Michael Noll, writing in Signals: The Science of Telecommunications, says a three minute coast to coast call cost $22.20. That's $411.47 in 2004 dollars.

In 1919 Theodore Gary and Company bought the Automatic Electric Company. Years later, when A.E. became AG Communication Systems, the AGCS website said "Theodore Gary aimed to cash in on the accelerating trend of replacing manual labor with machinery, and saw great potential in the Bell System market. Gary formed a syndicate that secured an option on the majority of Automatic Electric Company common stock. In 1919, he exercised his option to purchase the company."

Since Automatic Electric didn't manufacture for the Bell System the words "potential in the Bell System market" means licensing potential. Indeed, the AGCS site goes on to say that, "By the mid-1920s, AE was licensing about 80 percent of the automatic telephone equipment in the world. It became the second largest telecommunications manufacturer in the United States after Western Electric."

Finally, on November 8, 1919, in what must have been a humiliating experience for the telecommunications giant, AT&T at last introduced large scale automatic switching equipment to their telephone system. Using step by step equipment made, bought, and installed by Automatic Electric. The cut over to dial in Norfolk, Virginia was a complete Bell System policy change. No longer would they convert automatic dial systems to manual as they bought independent telephone companies, but they would instead embrace step by step equipment and install more.

More on the many mergers of Automatic Electric is here

In 1921 the Bell System introduced the first commercial , a very odd invention. Developed over eight years, it was AT&T's response to the automatic dialing feature offered by step by step equipment. It offered many innovations and many problems. Although customers could dial out themselves, the number of parts and its operating method made it noisy for callers. Ironically, some switchmen say it was a quiet machine inside the central office, emanating "a collection of simply delightful 'clinking,' 'whirring' and 'squeak, squeak, squeak' noises." Working like a game of Snakes and Ladders, the switch used selectors to connect calls, these mechanical arms moving up and down in large banks of contacts. When crossbar switching came on the scene in 1938, panel switches were removed where possible, although some remained working until the mid 1970s. Panel became the first defunct switch in the public switched telephone network.

At this site were marvelous photos of the last functional panel switch: http://xy3.com/phone/vintage/panel%201.stm (external link). If you have the time, you might try entering the URL above into the Internet Archive Wayback Machine (external link)

For a wonderful history of early electronic pioneering, click here for a must read interview with Ray Sears: http://www.ieee.org/organizations/history_center/oral_histories/transcripts/sears.html (external link)

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Part C

In 1925 Western Electric sold its overseas manufacturing plants to a small company with a big name and even bigger ideas: International Telephone and Telegraph. A controversial decision within the Bell System. AT&T sold factories in 11 countries, fearing a United States anti-trust lawsuit. Western kept a minority interest in one foreign company, Northern Electric, in Canada, until 1963.. AT&T would not return officially to the international market until 1977. [Kimberlin]

"Western Electric never controlled Northern Electric (now ) although they owned shares always in a minority position, the most they held was 43.57% in 1929, by 1962 they held .01% and by 1964 they were fully divested. The majority shareholder was the Bell Telephone Company of Canada." Thanks to Ken Lyons, Curator, Telecommunications Museum Telecommunications Museum,Maison des benevoles retraites, Nortel Retirees Club in Montreal, LaSalle, QC

ITT's owners, the curious, conspiratorial Behn brothers, Sosthenes and Hernand, bought Western Electric International for 30 million dollars and renamed it International Standard Electric. Their purchase, backed by J.P. Morgan's bank, included Western's large British manufacturer, renamed Standard Telephones and Cable. The Behns agreed not to compete in America against Western Electric, and to be the export agent for AT&T products abroad. AT&T agreed in return not to compete internationally against the Behns. Now equipped with a large manufacturing arm, IT&T spread across the globe, buying and influencing telephone companies (and their governments) on nearly every continent.

In January, 1927, commercial long distance radio-telephone service was introduced between the United States and Great Britain. AT&T and the British Postal Office got it on the air after four years of experimenting. They expanded it later to communicate with Canada, Australia, South Africa, Egypt and Kenya as well as ships at sea. This service had fourteen dedicated channels or frequencies eventually assigned to it. The overseas transmitter was at Rugby, England, and the United States transmitter was at Deal, New Jersey. (According to Bell Labs, but see Kimberlin's notes here.)[BLR] Nearly thirty years would pass before the first telephone cable was laid under the Atlantic, greatly expanding calling capacity. In the next year The Great Depression began, hitting independent telephone companies hard, including the manufacturer Automatic Electric.

Click here for an excellent discussion (internal link) of British involvement with radio telephone, by Don Kimberlin, and a photograph of the main transmitting tube at Rugby, a ten foot tall, one ton valve.

A photograph of AT&T's overseas radio-

Although telephones had been used in the White House for many years, the instrument did not reach the president's desk until the Hoover administration at the start of the Great Depression. "In 1929, when the Executive Offices were remodeled the historic one-position switchboard which had served for so many years was retired from service and a new two-position switchboard, especially built to meet the President's needs, was installed. The number of stations was materially increased in addition to many special circuits for the use of the President. It was at this time a telephone was installed on the President's desk for the first time." [Hoover Library]

(Thanks to L. Nickel for researching this point)

The United States Congress created the Federal Communications Commission in 1934 to regulate telephones, radio, and television. It was part of President Roosevelt's "New Deal" plan to bring America out of the Great Depression. Not content to merely follow congressional dictates, and unfortunately for wireless users, the agency first thought it should promote social change through what it did. To promote the greater good with radio, the F.C.C. gave priority to emergency services, broadcasters, government agencies, utility companies, and other groups it thought served the most people while using the least . This meant few channels for radio- telephones since a single wireless call uses the same as an F.M. radio broadcast station; large frequency blocks to serve just a few people.

Treating radio like a public utility, something like the railroads, it was thought a public agency could protect the public against monopoly practices and price gouging. But like many bureaucracies, at every opportunity the FCC tried to enlarge its role and power, eventually aligning itself with large communications companies and then actually working against the consumer. The worst examples were outside of telephony, helping the RCA corporation against F.M. , ruining Edwin Armstrong in the process, and favoring RCA over Farnsworth, the first real developer of television, leaving him penniless as well. Along the way were maddening delays in approving technical advances and frequency allocations, something that continues to this day.

Late in 1934 the FCC began investigating AT&T as well as every other telephone company. The FCC issued a 'Proposed Report' after four years, in which its commissioner excoriated AT&T for, among other things, unjustifiable prices on basic phone service. The commissioner also urged the government to regulate prices the Bell System paid Western Electric for equipment, indeed, even suggesting AT&T should let other companies bid on Western Electric work. The Bell System countered each point of the FCC's report in their 1938 Annual Report, however, it was clear the government was now closely looking at whether the Bell System's structure was good for America. At that time AT&T controlled 83 percent of United States telephones, 91 percent of telephone plant and 98 percent of long distance lines. Only the outbreak of World War II, two and a half months after the final report was issued in 1939, staved off close government scrutiny.

In 1937 Alec Reeves of Britain invented modern digital transmission when he developed Pulse Code . I say modern because Morse code and its variants are also digital: organized on and off pulses of electrical energy that convey information. While PCM took decades to implement, the advent of digital working was a momentous event and deserves much consideration. David Robertson, a biographer of Reeves, goes so far as to claim Reeves as the father of modern telecommunications. "I think a fair argument can be sustained that the adoption of digital is the principal motor of change in the early 21st century. For sure, there'd have been no merger between AOL and Time Warner and other moves towards combining media with telecom companies had it not become possible to transmit information of all sorts in the same binary way. Whether all this is good news is, of course, another issue."

For more information on Alec Reeves click here (internal link)

For a website devoted to Reeves go here: http://www.AlecHarleyReeves.com (external link)

In 1937 was installed between Toledo, Ohio and South Bend, Indiana. Long distance lines began moving underground, a big change from overhead lines carried on poles. In that same year the first commercial messages using carrier techniques were sent through the coax, based on transmission techniques invented by Lloyd Espenschied and Herman A. Affel. Multiplexing let toll circuits carry several calls over one cable simultaneously. It was so successful that by the mid 1950s seventy nine percent of Bell's inter city trunks were multiplexed. The technology eventually moved into the local network, improving to the point where it could carry 13,000 channels at once.

For more information on Lloyd Espenchied's brilliant career, go here: http://www.ieee.org/organizations/history_center/oral_histories/transcripts/espenschied11.html

In 1938 retractile, spring, or spiral cords were introduced into the Bell System. A single cotton bundle containing the handset's four wires were fashioned into a spiral. This reduced the twisting and curling of conventional flat or braided cords. Spiral cords were popular immediately. AT&T's Events in History [ETH] reported that introduction began in April, with Western Electric providing 6,000 cords by November. Still, even with W.E. then producing 1,000 cords a week, the cords could not be kept in stock.

In 1938 the Bell System introduced crossbar switching to the central office, a system as excellent as the panel switch was questionable. The first No. 1 crossbar was cut into service at the Troy Avenue central office in Brooklyn, New York on February 13th.This culminated a trial begun in October 1937. [ETH]A detail of a is shown on the right. Western Electric's models earned a worldwide reputation for ruggedness and flexibility. AT&T improved on work done by the brilliant Swedish engineer Gotthilf Ansgarius Betulander. They even sent a team to to look at his crossbar switch. Installed by the hundreds in medium to large cities, crossbar technology advanced in development and popularity until 1978, when over 28 million Bell system lines were connected to one. That compares to panel switching lines which peaked in 1958 at 3,838,000 and step by step lines peaking in 1973 at 24,440,000.

Much telephone progress slowed as World War II began. But one major accomplishment was directly related to it. On May 1, 1943 the longest open wire communication line in the world began operating between Edmonton, Alberta and Fairbanks, . Built alongside the newly constructed Alaskan Highway, the line was 1500 miles long, used 95,000 poles and featured 23 manned repeater stations. Fearing its radio and submarine cable communications to Alaska might be intercepted by the Japanese, the United States built the line to provide a more secure communication link from Alaska to the United States. A little bit on radar development in World War II

Back to crossbar. Note the watch-like complexity in the diagram. Current moving through the switch moved these electro-mechanical relays back and forth, depending on the dial pulses received. Despite its beauty, these switches were bulky, complicated and costly. The next invention we look at would in time sweep all manual and electro-mechanical switching away.

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Resources

[BLR] "The Opening of Transatlantic Service on Shortwaves" 6 Bell Laboratories Record 1928: 405

[Hoover Library] Personal correspondence from the Hoover Library to L. Nickel (10/19/2000)"

[ETH] Events in Telecommunication History, AT&T Archives Publication: Warren, New Jersey (8.92-2M), p53

[Kimberlin] "While AT&T purported to stay out of international markets, they always had an entity with several names like "Bell International" that functioned as sales offices to those who would inquire. No special modifications done, even for AC power. You take it the way we make it if you want it, was the slogan. To that extent, many of the overseas HF radio-telephone points we worked from ATT's Fort Lauderdale office had Western Electric HF and terminals that matched ours. There are many more stories like this . . ." Don Kimberlin (internal link to Don's page at this site).

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Part D On July 1, 1948 the Bell System unveiled the transistor, a joint invention of Bell Laboratories scientists William Shockley, , and Walter Brattain. It would revolutionize every aspect of the telephone industry and all of communications. One engineer remarked, "Asking us to predict what transistors will do is like asking the man who first put wheels on an ox cart to foresee the automobile, the wristwatch, or the high speed generator." Others were less restrained.

In 1954, recently retired Chief of Engineering for AT&T, Dr. Harold Osborne, predicted, "Let us say that in the ultimate, whenever a baby is born anywhere in the world, he is given at birth a number which will be his for life. As soon as he can talk, he is given a watchlike device with 10 little buttons on one side and a screen on the other. Thus equipped, at any time when he wishes to talk with anyone in the world, he will pull out the device and punch on the keys the number of his friend. Then turning the device over, he will hear the voice of his friend and see his face on the screen, in color and in three dimensions. If he does not see and hear him he will know that the friend is dead." [Conly]Sheesh.

The first transistor looking as crude, perhaps, as the first telephone. The point contact transistor pictured here is now obsolete. Capitalizing on a flowing stream of electrons, along with the special characteristics of silicon and germanium, the transistor was built into amplifiers and switching equipment. Hearing aids, radios, phonographs, , electronic telephone switching equipment, satellites and moon rockets would all be improved or made possible because of the transistor. Let's depart again from the narrative to see how a transistor works.

Transistor stands for transit resistor, the temporary name, now permanent, that the inventors gave it. These semidconductors control the electrical current flowing between two terminals by applying voltage to a third terminal. You now have a minature switch, presenting either a freeway to electrons or a brick wall to them, depending on whether a signal voltage exists. Bulky mechanical relays that used to switch calls, like the crossbar shown above, could now be replaced with transistors. There's more.

Transistors amplify when built into a proper circuit. A weak signal can be boosted tremendously. Let's say you have ten watts flowing into one side of the transistor. Your current stops because silicon normally isn't a good conducter. You now introduce a signal into the middle of the transistor, say, at one watt. That changes the transistor's internal crystalline structure, causing the silicon to go from an insulator to a conductor. It now allows the larger current to go through, picking up your weak signal along the way, impressing it on the larger voltage. Your one watt signal is now a ten watt signal.

Transistors use the properties of semi-conductors, seemingly innocuous materials like geranium and now mostly silicon. Materials like silver and copper conduct electricity well. Rubber and porcelain conduct electricity poorly. The difference between electrical conductors and insulators is their molecular structure, the stuff that makes them up. Weight, size, or shape doesn't matter, it's how tightly the material holds on to its electrons, preventing them from freely flowing through its atoms.

Silicon by itself is an ordinary element, a common part of sand. If you introduce impurities like arsenic or boron, though, you can turn it into a conductor with the right electrical charge. Selectively placing precise impurities into a silicon chip produces an . It's like making a magnetically polarized, multi-layered chemical cake. Vary the ingredients or elements and you can make up many kinds of cakes or transistors. And each will taste or operate a little differently.

As I've just hinted, there are many kinds of transistors, just as there are many different kinds of tubes. It's the triode's solid state equivalent: the field effect transistor or FET. The FET we'll look at goes by an intimidating name, MOSFET for Metal Oxide Field Effect Transistor. Whew! That's a big name but it describes what it does: a metal topped device working by a phenomenon called a field effect.

A silicon chip makes up the FET. Three separate wires are welded into different parts. These electrode wires conduct electricity. The source wire takes current in and the drain wire takes current out. A third wire is wired into the top. In our example the silicon wafer is positively charged. Further, the manufacturer makes the areas holding the source and drain negative. These two negative areas are thus surrounded by a positive. A much more accurate transistor explanation here

Now we introduce our weak signal current, say a that needs amplifying. The circuit is so arranged that its current is positive. It goes into the gate where it pushes against the positive charge of the silicon chip. That's like two positive magnets pushing against each other. If you've ever tried to hold two like magnets together you know it's hard to do -- there's always a space between them. Similarly, a signal voltage pushing against the chip's positive charge gives space to let the current go from the source to the drain. It picks up the signal along the way. Check out this diagram, modified only slightly from 's excellent site: http://www.lucent.com/minds/transistor/tech.html

As Louis Bloomfield of Virginia puts it:"The MOSFET goes from being an insulating device when there is no charge on the gate to a conductor when there is charge on the gate! This property allows to amplify signals and control the movements of electric charge, which is why MOSFETs are so useful in electronic devices such as stereos, , and computers."

I know that this is a simple explanation to a forbiddingly difficult topic, but I think it's enough for a history article. Thanks to Australia's John Wong for help with his section. If you'd like to read further, check out Lucent's transistor page by searching their site: http://www.lucent.com (external link) If you have a better explanation or something to add, please contact me And now back to the narrative.

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Part E

We come to the 1950s. was not widespread until the end of the decade in North America, not until direct dialing and automatic switching became common. Dial tone was first introduced into the public switched telephone network in a German city by the Siemens company in 1908, but it took decades before being accepted, with the Bell System taking the lead. AT&T used it not only to indicate that a line was free, but also to make the dialing procedure between their automatic and manual exchanges more familiar to their customers. Manual exchange subscribers placed calls first through an operator, who listened to the number the caller wanted and then connected the parties together. The Bell System thought dial tone a good substitute for an operator's "Number please" and required this service in all of their automatic exchanges. Before the 1950s most of the independent telephone companies, but not all, also provided dial tone. And, of course, dial tone was not possible on phones such as crank models, in which you signaled an operator who then later connected your call. [Swihart]

I mentioned direct number dialing, where callers made their own long distance calls, This was first introduced into the Bell System in a trial in Englewood, New Jersey in 1951. Ten years passed before it became universal.

On August, 17, 1951 the first transcontinental microwave system began operating. [Bell Laboratories Record] One hundred and seven relay stations spaced about 30 miles apart formed a link from New York to San Francisco. It cost the Bell System $40,000,000; a milestone in their development of radio relay begun in 1947 between New York and Boston. In 1954 over 400 microwave stations were scattered across the country. A Bell System "Cornucopia" tower is shown at left. By 1958 microwave carrier made up 13,000,000 miles of telephone circuits or one quarter of the nations long distance lines. 600 conversations or two television programs could be sent at once over these radio routes. But what about crossing the seas? Microwave wasn't possible over the ocean and radiotelephony was limited. Years of development lead up to 1956 when the first transatlantic telephone cable system started carrying calls. It cost 42 million dollars. Two coaxial cables about 20 miles apart carried 36 two way circuits. Nearly fifty sophisticated repeaters were spaced from ten to forty miles along the way. Each vacuum tube repeater contained 5,000 parts and cost almost $100,000. The first day this system took 588 calls, 75% more than the previous ten days average with AT&T's transatlantic radio-telephone service.

In the early 1950s The Bell System developed an improved neoprene jacketed telephone cord and shortly after that a PVC or plastic cord. [BLR.] These replaced the cotton covered cords used since telephony began. The wires inside laid parallel to each other instead of being twisted around. That reduced diameter and made them more flexible. Both, though, were flat and non- retractable, only being made into spring cords later. In the authoritative Dates in American Telephone Technology, C.D. Hanscom, then historian for Bell Laboratories, stated that the Bell System made the neoprene version available in 1954 and the plastic model available in 1956. These were, the book dryly indicated, the most significant developments in cord technology since 1926, when solderless cord tips came into use.

On June 7, 1951, AT&T and International Telephone and Telegraph signed a cross-licensing patent agreement. [Myer] This marks what Myer says "led to complete standardization in the American telephone industry." Perhaps. I do know that ITT's K-500 phones are completely interchangeable with W.E. Model 500s, so much so that parts can be freely mixed and matched with each other. But whether Automatic Electric and other manufacturers produced interoperable equipment is something I am still researching. [William Myre discussion on interchangeable parts]

It is significant, though, that after seventy-five years of competition the Bell System decided to let other companies use its patents. Myer suggests a 1949 anti-trust suit against WECO and AT&T was responsible for their new attitude. On August 9, 1951 ITT began buying Kellogg stock, eventually acquiring the company. In 1952 the Kellogg Switchboard and Supply company passed into history, merging with ITT.

Roger Conklin relates, "In just a few years after the buyout, ITT changed the name from Kellogg Switchboard & Supply Company to ITT Kellogg. Then, after merging Federal Telephone and Radio Corporation, its separate telephone manufacturing company in Clifton, NJ. into ITT Kellogg and combining manufacturing operations into its Cicero Ave. facility in Chicago, the name was changed again to ITT Telecommunications. . . . The last change to ITT Telecommunications [took] place [in]1963."

"In 1989, ITT sold its entire worldwide telecommunications products business to Alcatel and withdrew totally from this business. In 1992 Alcatel sold what had formerly been ITT's customer premises equipment (CPE) business in the US, including its factory in Corinth, MS. to a group of private investors headed by David Lee. Initially after purchasing this business from Alcatel, this new company was known as Cortelco Kellogg. It continues to manufacture and market what had formerly been ITT's U.S.-made telephones and related products. The name 'Kellogg' has since been dropped from its name and the company is now known as Cortelco. For a short while Cortelco continued to use the ITT name and trademark on its products under a license from ITT, but this also has been discontinued." The ITT information above came from the excellent history site http://www.sigtel.com/ (external link, now dead), produced by the U.K.'s Andrew Emmerson, a first rank telephone historian.

In 1952 the Bell System began increasing charges from a nickel to a dime. [Fagen] It wasn't an immediate change since both the payphone and the central office switching equipment that serviced it had to be modified. By the late 1950s many areas around the country were still charging a nickel. Most likely AT&T started converting in New York City first.

In the mid-50s Bell Labs launched the Essex research project. It concentrated on developing controlled switching, based upon using the transistor. It bore first fruit in November, 1963 with the 101 ESS, a PBX or office telephone switch that was partly digital. Despite their computer expertise, AT&T agreed in 1956 under government pressure not to expand their business beyond telephones and transmitting information. Bell Laboratories and Western Electric would not enter such fields as computers and business machines. In return, the Bell System was left intact with a reprieve from anti-monopoly scrutiny for a few years. It is interesting to speculate whether IBM would have dominated computing in the 1960s if AT&T had competed in that market.

In 1955 Theodore Gary and Company merged into General Telephone, forming the largest independent telephone company in the United States. The combined company served "582,000 domestic telephones through 25 operating companies in 17 states. It also had interests in foreign telcos controlling 426,000 telephones." Automatic Electric, Gary's most well known company, retained its name but fell under an even larger corporate umbrella. AGCS goes on to say,

The Gary merger package included Automatic Electric Co. (AE), which now had subsidiaries in Canada, Belgium and Italy. GTE had purchased its first telephone-manufacturing subsidiary five years earlier in 1950 - Leich Electric. But the addition of AE's engineering and manufacturing capacity assured GTE of equipment for their rapidly growing telephone operations.

An excellent timeline on Automatic Electric history is at the AGCS site. The "A" in the name stands for AT&T, the "G" for "GTE". Divisions from both companies combined in 1989 to form AGCS: http://www.agcs.com/ (external link)

General was founded in 1926 as Associated Telephone Utilities by Sigurd Odegard. The company went bankrupt during the Great Depression and in 1934 reorganized itself as General Telephone. General had its own manufacturing company, Leich Electric, which began in 1907. Growth was unspectacular until Donald C. Power became president in 1950. He soon bought other companies, building General Telephone into a large telecommunications company.

After the merger of Automatic Electric, General acquired producer Electric Secretary Industries in 1957, carrier equipment maker Lenkurt Electric in 1959, and Sylvania Electronics in that same year. In 1959 the newly renamed General Telephone and Electronics provided everything the independent telephone companies might want. Although they were not the exclusive manufacturer for the independents, Automatic Electric was certainly the largest. And where GTE aggressively went after military contracts, the Bell System did not. In the late 1950s, for example, Lenkurt Electric produced most of the armed forces' carrier equipment. GTE lasted until 1982.

In January, 1958, Wichita Falls, Texas was the first American city in the Bell System to institute true number calling, that is, seven numerical digits without letters or names. Although it took more than fifteen years to implement throughout the Bell System, ANC, or all number calling, would finally replace the system of letters and numbers begun forty years before at the advent of automatic dialing. Telephone numbers like BUtterfield8, ELliot 1-1017 or ELmwood 1-1017. For a history of exchange names, please click here to read my article on them. Keep in mind, too, that many independent telephone companies did not use letters and numbers,

For a history of country codes, all number dialing that let people call overseas on their own, click here: http://mirror.lcs.mit.edu/telecom-archives/archives/country.codes/ (external link)

For a look at the overwhelming subject of American area codes, go here: http://mirror.lcs.mit.edu/telecom-archives/archives/areacodes/ (external link)

The 1960s began a dizzying age of projects, improvements and introductions. In 1961 the Bell System started work on a classic cold war project, finally completed in 1965. It was the first coast to coast atomic bomb blast resistant cable. Intended to survive where the national microwave system might fail, the project buried 2500 reels of coaxial cable in a 4,000 mile long trench. 9300 circuits were helped along by 950 buried concrete repeater stations. Stretched along the 19 state route were 11 manned test centers, buried 50 feet below ground, complete with air filtration, living quarters and food and water. In 1963 the first modern touch-tone phone was introduced, the Western Electric 1500. It had only ten buttons. Limited service tests had started in 1959.

Also in 1963 digital carrier techniques were introduced. Previous multiplexing schemes used analog transmission, carrying different channels separated by frequency, much like those used by . T1 or Transmission One, by comparison, reduced analog voice traffic to a series of electrical plots, binary coordinates to represent sound. T1 quickly became the backbone of long distance toll service and then the primary handler of local transmission between central offices. The T1 system handles calls throughout the telephone system to this day.

In 1964 the Bell System put its star crossed videotelephone into limited commercial service between New York, Washington and Chicago. Despite decades of dreaming, development and desire by Bell scientists, technicians and marketing wonks, the videotelephone never found a market.

1968. Even the astute Japanese fell victim to developing picturephones as this unflattering photograph shows, this model was probably developed by Nippon Telephone and Telegraph

In 1965 the first commercial was launched, providing 240 two way telephone circuits. Also in 1965 the 1A1 payphone was introduced by Bell Labs and Western Electric after seven years of development. Replacing the standard three slot payphone design, the 1A1 single slot model was the first major change in coin phones since the 1920s.

1965 also marked the debut of the No. 1ESS, the Bell Systems first central office computerized switch. The product of at least 10 years of planning, 4,000 man years of research and development, as well as $500 million dollars in costs, the first Electronic Switching System was installed in Succasunna, N.J. Built by Western Electric the 1ESS used 160,000 diodes, 55,000 transistors and 226,000 resistors. These and other components were mounted on thousands of circuit boards. Not a true digital switch, the 1ESS did feature , a fancy Bell System name for memory, enabling all sorts of new features like speed dialing and . Without memory a switch could not perform these functions; previous switches such as crossbar and step by step worked in real time, with each step executed as it happened. The switch proved a success but there were some problems for Bell Labs engineers, particularly when a No.1ESS became overloaded. In those circumstances it tended to fail all at once, rather than breaking down bit by bit.

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Resources

[Myers] Myer, Ralph O, 1995, Old Time Telephones!: Technology, Restoration and Repair, Tab Books, New York. 123 Excellent.

[Swihart, Stanley] Telecom History: The Journal of the Telephone History Institute, Issue 2, Spring 1995

[ETH] Events in Telecommunication History, 1992 ,AT&T Archives Publication (8.92-2M), p53

[Bell Laboratories Record] "Coast to Coast Radio Relay System Opens." Bell Laboratories Record, May, 1951. 427

[Bell Laboratories Record] Weber, C.A., Jacketed Cords for Telephones, Bell Laboratories Record, May, 1959 187

[Fagen] Fagen, M.D., ed. A History of Engineering and Science in the Bell System: Volume 1 The Early Years, 1875 -1925. New York: Bell Telephone Laboratories, 1975, 357 Briefly mentions coin services.

[William Myre discussion on interchangeable parts]

As a teenager in the 60's, I did a detailed examination of both our Western Electric keyed telephones (installed in 1960) and a couple of Automatic Electric phones (on of which was keyed). All the phones were dial telephones. At the time I was attempting to understand the wiring and reverse engineer the circuitry.

It is my opinion that the parts were not designed to be mechanically interchangeable. The inside of the phones were laid out differently. The dial on a WE seemed to be different from a AE mechanically.

The electrical "guts" of both the WE and AE phones was a metal box with a plastic top on which screw terminals were located. The layout of these terminals and the box size was not the same.

The handset had dimensional differences as well, although the AE and WE mic and speaker might fit interchangeably.

Electrically speaking, of course, all phones had pretty much the same circuits and components, so it would probably be possible to wire a AE circuit box into a WE phone, and it likely work. The electrical differences, if they exist, would have to be in the microphone, speaker, or capacitor used in series with the ringer coil (and the coil impedance).

I don't remember if the same color coding was used on the internal wiring, but I can certainly say that having a WE phone to examine did not help me re-wire the inside of an AE phone that had been unwired.

I still have a keyed AE phone in my garage. I also somewhere probably still have the technical bulletin AE sent me to rewire the AE phone.

William Myre

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Part F

In June 1968 the FCC allowed non Bell equipment to be legally attached to Bell System lines. Despite restrictions the Bell System would impose on such equipment, many companies started producing products to compete with Western Electric. In 1969 Microwave Communications International began transmitting business calls over their own private network between Saint Louis and Chicago. Bypassing Bell System lines, MCI offered cheaper prices. AT&T bitterly opposed this specialized common carrier service, protesting that Bell System's long distance rates were higher since they subsidized local phone service around the country. Still, MCI was a minor threat, economically. The real problem started a few years later when MCI tried to connect to the Bell System network.

At the end of the 1960s AT&T began experiencing severe customer service problems, especially in New York City. The reasons were many but most had to do with unforeseen demand, coupled with reduced maintenance. The Bell System fixed the problems but not without an attitude that embittered people by the millions. In Boettinger's pro-Bell System history, he recounts the troubles this way: "In 1969, unprecedented jumps in usage and demand caused service deterioration in several large cities. Huge and rapid injections of equipment and personnel trained in accelerated programs were required before quality levels were restored. The experience showed how vital telephones had become to modern life (when even persons on welfare were felt to need a phone) and how frustrations with breakdown led to aggressive behavior." That the Bell System didn't understand how vital telephones were to modern life is beyond understanding; that welfare recipients weren't thought to deserve a phone is beyond acceptance, however, Ma Bell was not alone in dealing with dissatisfied customers. GTE also had problems.

GTE and Automatic Electric went through tremendous growth in the 1960s, with A.E. expanding to four different facilities. In 1969 their California facility in San Carlos made transmission equipment. Switchgear and related equipment came from Northlake and Genoa, Illinois, and telephones and other customer apparatus came from Huntsville, Alabama. Automatic Electric Limited in Canada also produced equipment. A.E.'s research in the 1960s resulted in their first computerized switch being cut into service into Saint Petersburg, Florida in September 1972. It was called the No. 1 EAX (Electronic Automatic Exchange). Growth wasn't handled well, though, by their parent company, General Telephone and Electronics.

GTE was then a poorly managed conglomerate of 23 regional phone companies and a maker of, among other things, televisions and light bulbs. They had their successes and failures. One notable achievement is below.

"Introducing a crimestopper so advanced Dick Tracy doesn't have it yet."

In1971 General Telephone and Electronics (GTE Sylvania) introduced a data system called Digicom. It let dispatchers identifying patrol car locations on a screen, and allowed officers to run license plate checks. When a patrolman touched a spot on the digicom screen it lit up the same spot on the dispatcher's map. Produced by their Sociosystems Products Organization, I do not know how many units were actually installed by GTE, but it certainly foreshadowed later developments. Today many police departments use cellular digital packet data (CDPD) to run plates and communicate in text with their dispatchers. CDPD runs on existing cellular networks, with data rates no more that 9.6 or 19.2 Kbs, adequate for most purposes but slow when you consider that in the year 2000, 29 years after this system was introduced, we are still laboring with creeping data rates. Click on the image above or here to get the full picture and story. (It's a huge graphic file so be careful: 364K)

GTE had their problems as well, especially with customer service, getting worse and worse through the late sixties, with the company admitting their problems by conducting a highly unusual national magazine ad campaign in November, 1971. The ad in the read:

"A lot of people have been shooting at the telephone companies these days. And, in truth, we've had our hands full keeping up with the zooming demand for increased phone service. But General Telephone and, in all fairness, the other phone companies haven't been sitting around counting dimes. For some time now, we've been paying a healthy 'phone bill' ourselves trying to make our service do everything you expect of it . . . During the next five years we'll be spending over $6 billion upgrading and expanding every phase of our phone operation . . . Ladies and gentlemen, we're working as fast as brains, manpower and money can combine to make our service as efficient as possible."

And although GTE might not have "sat around counting dimes," GTE's poor service record continued, a reputation that haunts it to this day. Rightly or wrongly, the phone companies, particularly those in the Bell System, watched agog as customer relations got worse. Hacking and toll fraud increased dramatically, as the phone company became fair game, a soulless and uncaring monster to war against. Attacking Ma Bell became common and almost fashionable.

1972 Mad Magazine cartoon. The caption reads: "Stockholders Grow FAT As Telephone Users Go Mad As Rates Rise And Service Flops."

In 1974 the Justice Department began investigating AT&T again for violating antitrust laws. They recommended Western Electric and Long Lines be divested from AT&T. Many people in Justice as well as throughout the country were concerned with the size of AT&T and their monopoly status. Although everyone knew the Bell System provided the best telephone service in the world, it had done so with little or no competition. AT&T's assets stood at 75 billion dollars. Big was not good in the early 1970s, with anti-establishment (particularly the military industrial establishment) feeling running high during the Vietnam and Watergate era. Contributing to the Bell System's woes, in July, 1977 the FCC instituted a certification program, whereby any telephone equipment meeting standards could be connected to Bell System's lines. Dozens and then hundreds of manufacturers started competing with Western Electric, making everything from answering machines, , machines, speakerphones, to differently styled telephones.

During the 1950s, 1960s, and 1970s, Stromberg-Carlson of Rochester, New York and then Lake Mary, Florida, produced a marvelously simple switch known as the X-Y. While an independent phone maker at the turn of the century, Stromberg-Carlson had by the early 70s been acquired by General Dyanmics. They were later bought by Rolm and then by Siemens of Germany, who still owns it today. It's new name is Siemens-Stromberg. But back to their switch. Little known outside the industry, the Stromberg-Carlson X-Y step by step switch solidly competed for business against Strowger technology (manufactured by Automatic Electric and others) in thousands of installations throughout rural America. Some may remain in Mexico and South America. Although the Bell System and many independents preferred the Strowger design for small communities, many telephone companies did not. Strowger equipment often worked reliably for decades but it was more complicated than X-Ys and it required a great deal of preventative maintenance performed by skilled craft workers. Ray Strackbein, who used to work for Stromberg-Carlson, says that X-Ys, by comparison, needed few repairs and fixes were simple. He writes, "I once met a husband-and-wife team that traveled throughout the Great Plains in their Winnebago motor home on a yearly cycle and routined hundreds of X-Y offices each year. They would work Arizona, New Mexico, and Texas in the winter, and Montana, Wyoming, and North Dakota in the summer. Even a Switchman who could not figure out how to wire a doorbell for a central office could maintain a C.O. full of X-Y switches."

Ray then goes on to describe the Stromberg-Carlson X-Y step by step switch, which could be configured or enlarged in blocks of 100 lines:

"Describing it is rough, but it was a modular switch that was horizontally slid into a vertical bay of shelves. An array of 400 (10X10X4) bare copper wires ran vertically behind the switch for the whole length of the bay. Four circuits were needed to make a connection: Tip, Ring, Sleeve, and Helper Sleeve.

Each switch sat on shelf about 12"X9"X2" (2" high). When someone dialed a number, the retracted switch moved horizontally -- the X direction -- (left-to-right as you faced it from the front), one step for each dial-pulse. Then when the dialed digit stopped pulsing, the switch rapidly extended horizontally away from you as you faced it, with four contacts, one for each circuit -- T, R, S, and HS -- sampling the 10 possible phone trunks for an idle trunk to the next selector.

The design of the X-Y switch was brilliant. Unlike the Strowger that lifted the armature for each dial pulse then rotating through a half-circle to find an idle line, the X-Y switch lifted no weight. The moving switch rested on the plate and moved only horizontally. This made for a switch of a much more simple design than the Strowger." [Strackbein] Please visit Ray Strackbien's site (external link)

Stromberg-Carlson introduced their first digital switch around 1978, the Stromberg Carlson System Century digital switch.

As switches were going digital, so, too, were nearly all electronics in the telecommunication field. Still, a few technological holdouts remained, as the Bell System replaced their last local cord switchboard in 1978, on Santa Catalina Island near the coast of Los Angeles, California. That's right, operators still placing calls by hand in the Age of Disco. "[T]he smallest version of Western's 160 toll switchboard" was replaced by a 3ESS, the first Bell switch, incidentally, to be shipped by barge. The city served would have been Avalon. This according according to the June, 1978 Bell Laboratories Record and personal correspondence with P. Egly of Santa Rosa, California.

Egly relates the following about Avalon:

"Tom, Avalon had its own inward operator and I even remember the route, 213 + 012 +... Calls off the island were handled by the same operator using She surely dialed all calls in the same way that any of the operators in the LA toll centers did. I am not sure if the trunks to the mainland were by microwave or by cable. "

"[Since this was a manual exchange] There were no dial phones on Avalon, all were manual magneto service with even the payphones having cranks. Most of the private subscribers had 300 or 500 type sets with dial blanks connected to magneto boxes. The operator rang the subscriber from her board using her ring key to supply ringing current from a standard WECO ring generator."

He goes on to say that the Bell System had a like system in Nevada:

"There was a similar situation in Virginia City, Nevada with the subscribers having the old walnut and oak magneto phones with local battery. In this case, most subscribers resisted the cutover to dial service, since the magneto phones were quite elegant. . . all polished wood and gleaming brass bells. They were part of the period atmosphere of the town."

This simple switching technology came within six years of outliving the most advanced telephone company on earth. But one manual local toll board remained in the public switched telephone network even longer.

J.R. Snyder Jr. reminds us that toll boards, manual long distance switches (internal link to article), were still working in the Bell System after the last local plug board was removed.

------

Resources

[Strackbein] Personal correspondence with Tom Farley, July 16, 2000. Another comment from Ray: "I didn't know that Strowger was from Penfield. That may partially explain why Stromberg- Carlson located in Rochester. As an aside, there is a building in Rochester called "Carlson Park" (as in industrial park). The parking lot looks just like it did in the mid '70's when I last saw it (I was teaching a class for Xerox in Webster last year and mentioned that I used to work for Stromberg and someone in the class said that the old plant was still there -- which it is -- so I drove over for a peek. The only difference is that they have sub-divided the administrative offices into private office suites and businesses.) Except for the new business signs, everything looks exactly as I remember it from 25 years ago."

[ETH] Events in Telecommunication History, 1992 ,AT&T Archives Publication (8.92-2M), p53

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Part G Michael Hathaway reports that "[My] parents owned the Bryant Pond Telephone Company in Bryant Pond, Maine, the last hand-crank magneto company to go dial. It was in our living room and the last call was made October 11, 1983." Hand crank magneto switchboards evolved around the turn of the century. Their arrangement was not common battery, where the exchange or central office powers their equipment and supplies electricity to customer's phones. Rather, as we saw earlier in this series, a crank at the switchboard operators position was turned to signal a customer. Turn the crank and you caused a dial at a customer's telephone to ring, a magneto in the crank generating the ringing current. To place a call a customer signaled the operator with a similiar crank on their telephone. A big battery in the base of the customer's telephone supplied the talking power when a call got connected. This system is called local battery, where the customer's phone supplies the power. Here's an example of a magneto switchboard below, a 1914 Western Electric Type 1200, known as a "Bull's Eye." This board is at the Roseville Telephone Company Museum and it still works for demonstrations. Click here or on the image below to see the large version.

So, you had many people on non-dial, candlestick or box telephones, as nearly a hundred years before. My father, incidentally, worked a magneto powered switchboard in his youth, near Davidson, Michigan. Mike goes on to say that,

"My father and mother Elden & Barbara Hathaway sold the Bryant Pond Telephone Company in 1981 but it took two years to convert. They did have about 400 customers ( probably 200 lines - two switchboards full). When they bought the company there were only 100 customers. The Oxford County Telephone Company, which bought it, retained ownership of the last operating switchboards, and they are currently deciding what they would like to do with them. The options include giving them to the town of Bryant Pond, and I have heard there is interest from the Smithsonian. My mother, who is 83, thinks that's quite exciting.

A lot of the family memorabilia has been donated to the Fryeburg Fair (Maine) Farm Museum, which although is only open during the 8 day fair, is visited by many thousands each year. It is hoped to have within a year or so a working magneto switchboard there where someone can call from an old pay phone to anywhere. My mother has a lot of telephone parts left over which we are slowly marketing for her as memorabilia from the last old hand-crank magneto company. I've actually written a book about the Bryant Pond Telephone Company called 'Everything Happened Around The Switchboard.' It's (obviously) a story of family life around the switchboard and is light reading with hopefully humor and nostalgia. I have lots of copies left and sell it directly. The address is Mike Hathaway, PO Box 705, Conway, NH 03818. But it is also available from Phonecoinc.com, and several bookstores."

This site has a great list of ending dates in telephonic history: http://www.sigtel.com/tel_hist_lasts.html

To sum up, although some manual switchboards may have remained in the PSTN, those being small office switches, or PBXs, the Bryant Pond board was the last central office manual exchange in America. On this happy and nostalgic note of technology passing away, so at the same time was the world's greatest telephone company coming to an end.

Although they had pioneered much of telecom, many people though the was growing faster than the Bell System could keep up. Many thought AT&T now stood in the way of development,

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Epilogue I: the death of Western Electric

"On January 1, 1984, the Western Electric Company, then older than the telephone itself, ceased to exist (Hochheiser 1991, 143). On that day of court ordered divestiture, the Bell System was broken into seven regional operating companies (the Baby Bells) and a more compact AT&T. AT&T retained the long-distance part of the business, its venerable research organization (Bell Laboratories), and its manufacturing operations (which could no longer have exclusive supply arrangements with the operating companies). A newly created AT&T Technologies, Inc. assumed the corporate charter of Western Electric and continued making 500-type,2500-type, and Trimline telephones under the AT&T Technologies label for several years at plants in Indianapois and Shreveport. However, to become competitive in the market, AT&T shifted residential telephone manufacturing to the Far East, beginning in Hong Kong in late 1985, Singapore the following year, and later in Bangkok and elsewhere. Thus ended U.S. production of rugged electromechanical telephones, and though phones similar to the 500-type, the 2500-type, the Princess, and the Trimline are still made to-day, they are products of the modern electronics age, rather than a bygone culture."

From: Old Time Telephones:Technology, Restoration and Repair by Ralph O Myer, Published by TAB Books, a division of McGraw Hill, Inc., Blue Ridge Summit, PA 17294 1 -800-822-8158 (717)-794-2191 (717)-794-2103 FAX ISBN No. 0-07-041817-9 (Paperback) 1995

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Epilogue II: A personal note on W.E.C.O. Yesterday I brought home a battered and rotten wooden crate I found outside a second hand store. I say outside because it was in such bad shape that not even the thrift store thought it saleable, they discarded it instead. Hardly fit as even a garden planter, I brought this oily and broken box home because of two words stenciled in three inch letters on the lengthwise sides: Western Electric. Gone are the rope handles and original hinges, and although the clasp appears genuine, it has been torn off once or twice and mounted in a new location each time. The stylized Bell System logo accompanies the lettering. There is an address on it. In handwriting that could only be penned by someone now in their 70s, the labeling reads, WECO, 1610 N. Broadway, Stockton, California. B/C 45738. I'm not sure if I will restore the box, put plants in it, or put the boards with the wording into a frame. It seems so sad and I keep thinking of the Ralph Myers' quote I used above. . .

I recommend Myers book to anyone who repairs or wants to understand old telephones.

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Epilogue III: Graham versus Gray

I haven't given my opinion directly as to who was first at the patent office, Gray or Bell. I'm not sure I can do it now, at least, not without being long winded. But let me try, in long sentences.

Detractors claim that the 600 court cases which followed the most valuable patent ever issued settled nothing. They say there was never any evidence that Bell did not cheat Gray. They try to prove a negative. They can't find any evidence that he cheated but they find nothing that absolutely clears Bell. He must have cheated.

But in his entire life of being a man and a humanist, for all his later works of invention, and contributions to charity, the founding of the National Geographic Society, his continued work with the deaf, in his voluminous note taking of all things scientific, in all of this, in this incredible record, there is absolutely nothing in Bell's character that suggests he was a cheat. Nothing. Nothing!

It is tough in this age of cynicism to admit that both Bell and Watson were truly great, gentle, brilliant men. Who deserved every bit of fame and accolade that came to them. Bell surrounded himself with sharp Boston lawyers to protect himself. But the animosity people had against his legal staff should in no way detract against Bell himself. Bell was an honest, courageous soul who long suffered being called a cheat. It was completely undeserved.

What about 1984 to the present? Read an excellent summary of technology development since the mid-1980s by Terry Edwards. It is a free .pdf file from his book Gigahertz and Terahertz Technologies for Communications (28 pages, 360K in .pdf)

Ordering information for this title (external link to Amazon)

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Why is there no "Q" or "Z" on many telephones?

This fascinating story came from http://www.LearningKingdom.com, now out of business.

Some voice mail systems don't take into account that not every phone has a Q or Z . . .

The telephone's pad of twelve buttons reflects its history. There are three letters on most buttons, except for zero, one, octothorp (#) and the star symbol (*), which have no letters. "Q" and "Z" are usually missing from the list. Why?

Instead of twelve buttons, telephones used to have circular plates with ten holes numbered from zero to nine. To make phone numbers easier to remember, the phone companies assigned letters to the numbers, so people could remember mnemonics like "Charleston" for C-H instead of the first two digits of a number. Of the ten digits, zero was already used to dial the operator and one was used for internal phone company signals. That left eight numbers to which letters could be assigned. Three letters per number took care of 24 of the alphabet's 26 letters, and the least common letters "Q" and "Z" were left out, but not forever. Many telephones now show "Q" on the seven button, and "Z" on the nine button.

Wither the ?

A comment from a reader: "The busy signal is going away . . ."

True; with voice mail and answering machines you don't get one. In 1995 The New Brunswick Telephone Company announced they would do away with busy signals for calls made within their territory. Instead of a busy signal callers got a recording which asked them to make one of three choices: send a message, for a price, hang up, or be notified when the line was available. Again, for a price. I wonder if anyone in that province misses the busy signal.

SBC/Pacific Bell offered this service in my area earlier this year, people hated it, I think because it was so aggressively pitched. Instead of getting a busy signal, a frustrating experience by itself, people got a come on, a promotion to buy something. If the Canadian telco didn't sell it too hard then perhaps people accepted it.

Since we haven't always had them so I shan't miss them when they go. They were an interlude only, although a longish one, good I should think for another decade or two. When calls were manually switched there was no need for a busy signal. An operator knew if a line was busy by looking at a lamp or a marker, what was called a drop, on a manual switch board. The operator then told the caller the line was busy.

When dialing became automatic network progress tones such as dial tone and busy signals were needed to tell the subscriber the status of a call. There is another busy signal, of course, that one being a "fast busy" signal, going at twice the rate of the normal tone. It indicates that telephone company circuits are too busy to handle a call. Not often heard on phones but quite common on cellular telephone networks.

Voice mail and answering machines and call waiting are, I suppose, just automatic operators, a step up above the obnoxious busy signal and of course quite a few steps below that of a real person to take a message. Although their people don't switch calls, perhaps answering services for doctors and lawyers are the last remnant of the always present, human attended exchange.

Did Alexander Graham Bell help dispel the ether theory?

Did Alexander Graham Bell help dispel the ether theory? And how much did it cost him? The answers are yes, and 200 bucks. The fascinating reading below is from Science in American Society: A Social History by George H. Daniels, 1971, Borzoi Books, Alfred Knoph:

"In 1881, a young American physicist then studying in Germany received a grant of $200 from Alexander Graham Bell to conduct an experiment on one of the most fascinating questions of nineteenth-century physics: the reality of the ether. The ether was a mysterious, jellylike, invisible entity which was thought to fill all of space; it was even present in solid matter. The vibrations set up in this ether made it possible to explain how the wavelike radiations of light could be carried through millions of miles without weakening or diluting their initial energy. Although the behavior of light seemed to demand some such medium, Albert A. Michelson doubted its existence, and he designed a relatively simple experiment which he thought might resolve the question unconditionally."

"With his $200 provided by Bell, Michelson had a machine of his own design, called the interferometer, constructed by a Berlin manufacturer, and he took it to the observatory at Potsdam for the crucial experiment. His conclusion, published in the August I88I issue of the American Journal of Science, was that 'the hypothesis of a stationary ether is erroneous.' Although Michelson later repeated the experiment, with more sophisticated apparatus, in collaboration with Edward Williams Morley it was the first experiment which, as Albert Einstein remarked, 'showed that a profound change of the basic concepts of physics was inevitable' and led eventually to Michelson's becoming the first American recipient of a Nobel prize." Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions. Basic Call Progress

The progress of a telephone call with loop-start signaling in place can be divided into five phases; on-hook, off-hook, dialing, switching, ringing, and talking. Figure 1 shows the on-hook phase.

Figure 1

When the handset rests on the cradle, the circuit is on-hook. In other words, before a phone call is initiated, the telephone set is in a ready condition waiting for a caller to pick up its handset. This state is called on-hook. In this state, the 48-VDC circuit from the telephone set to the CO switch is open. The CO switch contains the power supply for this DC circuit. The power supply located at the CO switch prevents a loss of telephone service when the power goes out at the location of the telephone set. Only the ringer is active when the telephone is in this position. Figure 2 shows the off-hook phase.

Figure 2 The off-hook phase occurs when the telephone customer decides to make a phone call and lifts the handset from the telephone cradle. The switch hook closes the loop between the CO switch and the telephone set and allows current to flow. The CO switch detects this current flow and transmits a dial tone (350- and 440-hertz [Hz] tones played continuously) to the telephone set. This dial tone signals the customer can begin to dial. There is no guarantee that the customer hears a dial tone right away. If all the circuits are used, the customer could have to wait for a dial tone. The access capacity of the CO switch used determines how soon a dial tone is sent to the caller phone. The CO switch generates a dial tone only after the switch has reserved registers to store the incoming address. Therefore, the customer cannot dial until a dial tone is received. If there is no dial tone, then the registers are not available. Figure 3 shows the dialing phase.

Figure 3

The dialing phase allows the customer to enter a phone number (address) of a telephone at another location. The customer enters this number with either a rotary phone that generates pulses or a touch-tone (push-button) phone that generates tones. These telephones use two different types of address signaling in order to notify the telephone company where a subscriber calls: Dual tone multifrequency (DTMF) dialing and Pulse dialing.

These pulses or tones are transmitted to the CO switch across a two-wire twisted-pair cable (tip and ring lines). Figure 4 shows the switching phase.

Figure 4

In the switching phase, the CO switch translates the pulses or tones into a port address that connects to the telephone set of the called party. This connection could go directly to the requested telephone set (for local calls) or go through another switch or several switches (for long-distance calls) before it reaches its final destination. Figure 5 shows the ringing phase.

Figure 5 Once the CO switch connects to the called line, the swtich sends a 20-Hz 90V signal to this line. This signal rings the phone of the called party. While ringing the phone of the called party, the CO switch sends an audible ring-back tone to the caller. This ring-back lets the caller know that ringing occurs at the called party. The CO switch transmits 440 and 480 tones to the caller phone in order to generate a ring-back. These tones are played for a specific on time and off time. If the called party phone is busy, the CO switch sends a busy signal to the caller. This busy signal consists of 480- and 620-Hz tones. Figure 6 shows the talking phase.

Figure 6

In the talking phase, the called party hears the phone ringing and decides to answer. As soon as the called party lifts the handset, an off-hook phase starts again, this time on the opposite end of the network. The is closed on the called party side, so current starts to flow to the CO switch. This switch detects current flow and completes the voice connection back to the calling party phone. Now, voice communication can start between both ends of this connection.

Table 1 shows a summary of alerting tones that could be generated by the CO switch during a phone call.

Table 1

The progress tones in Table 1 are for North American phone systems. International phone systems can have a totally different set of progress tones. Everyone must be familiar with most of these call progress tones.

A Dial tone indicates that the telephone company is ready to receive digits from the user telephone.

A Busy tone indicates that a call cannot be completed because the telephone at the remote end is already in use.

A Ring-Back (normal or PBX) tone indicates that the telephone company is attempting to complete a call on behalf of a subscriber.

A Congestion progress tone is used between switches to indicate that congestion in the long distance telephone network currently prevents a telephone call from being progressed.

A Reorder tone indicates that all the local telephone circuits are busy, and thus prevents a telephone call from being processed. A Receiver off-hook tone is the loud ringing that indicates the receiver of a phone is left off- hook for an extended period of time.

A No such number tone indicates that the number dialed cannot be found in the routing table of a switch. Address Signaling and Tip and Ring

Address Signaling

North American Numbering Plan

The North American Numbering Plan (NANP) uses ten digits to represent a telephone number. These ten digits are divided into three parts: the area code, office code, and station code.

In the original NANP, the area code consisted of the first three digits of the telephone number and represented a region in North America (including Canada). The first digit was any number from 2 to 9, the second digit was 1 or 0, and the third digit was any number from 0 to 9. The office code consisted of the second three digits of the telephone number and uniquely identified a switch in the telephone network. The first digit was any number from 2 to 9, the second digit was any number from 2 to 9, and the third digit was any number from 0 to 9. The area code and office code could never be the same because the second digit of each code was always different. With this numbering system, the switch was able to determine whether this was a local call or long- distance call with the second digit of the area code. The station code consisted of the last four digits in the telephone number. This number uniquely identified a port within the switch that was connected to the telephone being called. Based on this ten-digit numbering system, an office code could have up to 10,000 different station codes. In order for a switch to have more than 10,000 connections, it has to have more office codes assigned to it.

An increase in the number of phone lines installed in homes, , and fax machine usage dramatically reduced the number of phone numbers available. This scenario prompted a change in the NANP. The present plan is basically the same as the old plan except for the area code and office code sections of the telephone number. The three digits for the area code and office code are now selected in the same fashion. The first digit can be any number from 2 to 9, and the second and third digits can be any number from 0 to 9. This scenario dramatically increases the number of area codes available, it in turn increases the number of station codes that can be assigned. If the call is a long-distance number, a one must be dialed before the 10-digit number.

International Numbering Plan

The International Numbering Plan is based on ITU-T specification E.164, an international standard that all countries must follow. This plan states that the telephone number in every country cannot be greater than 15 digits. The first three digits represent the , but each can choose whether to use all three digits. The remaining 12 digits represent the national specific number. For example, the country code for North America is 1. Therefore, when calling North America from another country, 1 must be dialed first in order to access the NANP. Then the ten digits required by the NANP are dialed. The 12 digits of the national specific number can be organized in any manner deemed appropriate by the specific country. Also, some countries can use a set of digits to indicate an outgoing . For example, 011 is used from within the United States to place an outgoing international call. Figure 7 illustrates network addressing in North America.

Figure 7

In this figure, the caller generates a call from within a customer premise that uses a PBX to access the Public Switched Telephone Network (PSTN). To get past the PBX, the caller must dial 9 first (this is how most PBXs are set up). Then, the caller must dial 1 for long distance and the ten-digit number of the telephone the caller wants to reach. The area code takes the caller through two switches, first a local switch and then an inter-exchange carrier (IXC) switch, which takes the call long distance. The office code (second three digits) takes the caller through a local switch again, and then to another PBX. Finally, the station code (last four digits) takes the caller to the telephone called.

Pulse Dialing

Pulse Dialing is an in-band signaling technique. It is used in analog telephones that have a rotary dialing switch. The large numeric dial-wheel on a rotary-dial telephone spins to send digits to place a call. These digits must be produced at a specific rate and within a certain level of tolerance. Each pulse consists of a "break" and a "make", which are achieved when the local loop circuit is opened and closed. The break segment is the time during which the circuit is open. The make segment is the time time during which the circuit is closed. Each time the dial is turned, the bottom of the dial closes and opens the circuit leading to the CO switch or PBX switch. A "governor" inside the dial controls the rate at which the digits are pulsed; for example, when a subscriber dials a digit on the rotary dial to call someone, a spring winds. When the dial is released, the spring rotates the dial back to its original position, and a cam-driven switch opens and closes the connection to the telephone company. The number of consecutive opens and closes--or breaks and makes-- represents the dialed digits Therefore, if the digit 3 is dialed, the switch is closed and opened three times. Figure 8 represents the sequence of pulses that occur when a digit 3 is dialed with pulse dialing.

Figure 8

This illustration displays the two terms, make and break. When the telephone is off-hook, a make occurs and the caller receives a dial tone from the CO switch. Then the caller dials digits, which generate sequences of makes and breaks that occur every 100 milliseconds (ms). The break and make cycle must correspond to a ratio of 60 percent break to 40 percent make. Then the phone stays in a make state until another digit is dialed or the phone is put back to an on-hook (equivalent to a break) state. Dial pulse addressing is a very slow process because the number of pulses generated equates to the digit dialed. So, when a digit 9 is dialed, it generates nine make and break pulses. A digit 0 generates ten make and break pulses. In order to increase the speed of dialing, a new dialing technique (DTMF) was developed. Figure 9 shows the frequency tones generated by DTMF dialing (also called touch-tone dialing).

DTMF Dialing

Figure 9 DTMF dialing is an in-band signaling technique just like pulse dialing. This technique is used in analog telephone sets that have a touch-tone pad. This dialing technique uses only two frequency tones per digit, as shown in Figure 9. Each button on the keypad of a touch-tone pad or a push- button telephone is associated with a set of high and low frequencies. On the keypad, each row of the key is identified by a low-frequency tone, and each column is associated with a high- frequency tone. The combination of both tones notify the telephone company of the number called, hence the term dual tone multifrequency. Therefore, when digit 0 is dialed, only frequency tones 941 and 1336 are generated instead of the ten make and break pulses generated by pulse dialing. The timing is still a 60-ms break and 40-ms make for each frequency generated. These frequencies were selected for DTMF dialing based on their insusceptibility to normal background noise.

Single-Frequency and Multifrequency Signaling

R1 and R2 signaling standards are used to transmit supervisory and address signaling information between voice network switches. They both use single-frequency signaling for transmission of supervisory information and multifrequency signaling for addressing information.

R2 Signaling

R2 signaling specifications are contained in ITU-T Recommendations Q.400 through Q.490. The physical connection layer for R2 is usually an E1 (2.048 megabits per second [Mbps]) interface that conforms to ITU-T standard G.704. The E1 digital facilities carrier runs at 2.048 Mbps and has 32 time-slots. E1 time-slots are numbered TS0 to TS31, where TS1 through TS15 and TS17 through TS31 are used to carry voice, which is encoded with pulse code modulation (PCM), or to carry 64 kbps data. This interface uses time slot 0 for synchronization and framing (same as for Primary Rate Interface [PRI]) and uses time slot 16 for ABCD signaling. There is a 16-frame multiframe structure that allows a single 8-bit time slot to handle the for all 30 data channels. R2 Call Control and Signaling

Two types of signaling are involved: line signaling (supervisory signals) and inter- (call setup control signals). Line signaling involves supervisory information (on-hook and off-hook) and inter-register signaling deals with addressing. These are described in more detail in this document.

R2 Line Signaling

R2 uses channel-associated signaling (CAS). This means that, in the case of E1, one of the time slots (channels) is dedicated to signaling as opposed to the signaling used for T1. The latter uses the top bit of every time slot in every sixth frame.

This signaling is out-of-band signaling and uses ABCD bits in a similar manner to T1 robbed-bit signaling to indicate on-hook or off-hook status. These ABCD bits appear in time slot 16 in each of the 16 frames that make up a multiframe. Of these four bits, sometimes known as signaling channels, only two (A and B) are actually used in R2 signaling; the other two are spare.

In contrast to robbed-bit signaling types such as wink start, these two bits have different meanings in the forward and backward directions. However, there are no variants on the basic signaling protocol.

Line signaling is defined with these types:

R2-Digital—R2 line signaling type ITU-U Q.421, typically used for PCM systems (where A and B bits are used).

R2-Analog—R2 line signaling type ITU-U Q.411, typically used for carrier systems (where a Tone/A bit is used).

R2-Pulse—R2 line signaling type ITU-U Supplement 7, typically used for systems that employ satellite links (where a Tone/A bit is pulsed).

R2 Interregister Signaling

The transfer of call information (called and calling numbers, and so on) is performed with tones in the time slot used for the call (called in-band signaling).

R2 uses six signaling frequencies in the forward direction (from the initiator of the call) and a different six frequencies in the backward direction (from the party who answers the call). These inter-register signals are of the multifrequency type with a two-out-of-six in-band code. Variations on R2 signaling that use only five of the six frequencies are known as decadic CAS systems. Inter-register signaling is generally performed end-to-end by a compelled procedure. This means that tones in one direction are acknowledged by a tone in the other direction. This type of signaling is known as multifrequency compelled (MFC) signaling.

There are three types of inter-register signaling:

R2-Compelled—When a tone-pair is sent from the switch (forward signal), the tones stay on until the remote end responds (sends an ACK) with a pair of tones that signals the switch to turn off the tones. The tones are compelled to stay on until turned off.

R2-Non-Compelled—The tone-pairs are sent (forward signal) as pulses, so they stay on for a short duration. Responses (backward signals) to the switch (Group B) are sent as pulses. There are no Group A signals in non-compelled inter-register signaling.

Note: Most installations use non-compelled inter-register signaling.

R2-Semi-Compelled—Forward tone-pairs are sent as compelled. Responses (backward signals) to the switch are sent as pulses. This scenario is the same as compelled, except that the backward signals are pulsed instead of continuous.

Features that can be signaled include:

 Called or calling party number  Call type (transit, maintenance, and so on)

 Echo-suppressor signals

 Calling party category

 Status

R1 Signaling

R1 signaling specifications are contained in ITU-T Recommendations Q.310 through Q.331. This document contains a summary of the main points . The physical connection layer for R1 is usually a T1 (1.544-Mbps) interface that conforms to ITU-T standard G.704. This standard uses the 193rd bit of the frame for synchronization and framing (same as T1).

R1 Call Control and Signaling

Again two types of signaling are involved: line signaling and register signaling. Line signaling involves supervisory information (on-hook and off-hook) and register signaling deals with addressing. Both are discussed in more detail:

R1 Line Signaling R1 uses in-slot CAS by bit robbing the eighth bit of each channel every sixth frame. This type of signaling uses ABCD bits in an identical manner to T1 robbed-bit signaling to indicate on-hook or off-hook status.

R1 Register Signaling

The transfer of call information (called and calling numbers, and so on) is performed with tones in the time slot used for the call. This type of signaling is also called in-band signaling.

R1 uses six signaling frequencies that are 700 to 1700 Hz in 200-Hz steps. These inter-register signals are of the multifrequency type and use a two-out-of-six in-band code. The address information contained in the register signaling is preceded by a KP tone (start-of-pulsing signal) and terminated by a ST tone (end-of-pulsing signal).

Features that can be signaled include:

 Called-party number  Call status

Tip and Ring Lines

Figure 10 illustrates tip and ring lines in a plain old telephone service (POTS) network.

Figure 10

The standard way to transport voice between two telephone sets is to use tip and ring lines. Tip and ring lines are the of wires that connect to your phone by way of an RJ-11 connector. The sleeve is the ground lead for this RJ-11 connector. Loop-Start Signaling

Loop-start signaling is a supervisory signaling technique that provides a way to indicate on-hook and off-hook conditions in a voice network. Loop-start signaling is used primarily when the telephone set is connected to a switch. This signaling technique can be used in any of these connections:

 Telephone set to CO switch  Telephone set to PBX switch

 Telephone set to foreign exchange station (FXS) module (interface)

 PBX switch to CO switch

 PBX switch to FXS module (interface)

 PBX switch to foreign exchange office (FXO) module (interface)

 FXS module to FXO module

Analog Loop-Start Signaling

Figures 11 through 13 illustrate loop-start signaling from a telephone set, PBX switch, or FXO module to a CO switch or FXS module. Figure 11 shows the idle state for loop-start signaling.

Figure 11

In this idle state, the telephone, PBX, or FXO module has an open two-wire loop (tip and ring lines open). It could be a telephone set with the handset on-hook, or a PBX or FXO module that generates an open between the tip and ring lines. The CO or FXS waits for a closed loop that generates a current flow. The CO or FXS have a ring generator connected to the tip line and – 48VDC on the ring line. Figure 12 shows an off-hook state for a telephone set or a line seizure for a PBX or FXO module.

Figure 12

In this illustration, a telephone set, PBX, or FXO module closes the loop between the tip and ring lines. The telephone takes its handset off-hook or the PBX or FXO module closes a circuit connection. The CO or FXS module detects current flow and then generates a dial tone, which is sent to the telephone set, PBX, or FXO module. This indicates that the customer can start to dial. What happens when there is an incoming call from the CO switch or FXS module? Figure 13 shows this situation.

Figure 13 In the illustration, the CO or FXS module seizes the ring line of the telephone, PBX, or FXO module called by superimposing a 20-Hz, 90-VAC signal over the –48VDC ring line. This procedure rings the called party telephone set or signals the PBX or FXS module that there is an incoming call. The CO or FXS module removes this ring once the telephone set, PBX, or FXO module closes the circuit between the tip and ring lines. The telephone set closes the circuit when the called party picks up the handset. The PBX or FXS module closes the circuit when it has an available resource to connect to the called party. The 20-Hz ringing signal generated by the CO switch is independent of the user lines and is the only way to let a user know that there is an incoming call. The user lines do not have a dedicated ring generator. Therefore, the CO switch must cycle through all the lines it must ring. This cycle takes about four seconds. This delay in ringing a phone causes a problem, known as glare, when the CO switch and the telephone set PBX, or FXO module seize a line simultaneously. When this happens, the person who initiates the call is connected to the called party almost instantaneously, with no ring-back tone. Glare is not a major problem from the telephone set to the CO switch because an occasional glare situation can be tolerated by the user. Glare becomes a major problem, when a loop-start is used from the PBX or FXO module to the CO switch or FXS module because more call traffic is involved. Therefore, the chance of glare increases. This scenario explains why loop-start signaling is used primarily when a connection is made from the telephone set to a switch. The best way to prevent glare is to use ground-start signaling, which is covered in a later section.

Digital Loop-Start Signaling for 26/36/37xx platforms

These diagrams show the bit status for ABCD bits for FXS/FXO loop-start signaling as it applies to 26/36/37xx platforms: Digital Loop-Start Signaling for AS5xxx

These diagrams show the bit status of AB bits for FXS/FXO loop-start signaling as it applies to only AS5xxx platforms. This is not applicable to 26/36/37xx platforms. This mode of operation is most commonly used in off-premise extension (OPX) applications. This is a two-state signaling scheme, using the "B bit" for signaling.

Idle Condition:

To FXS: A bit = 0, B bit = 1

From FXS: A bit = 0, B bit = 1 FXS Originates:

Step 1: FXS changes A bit to 1, signaling the FXO to close the loop.

To FXS: A bit = 0, B bit = 1

From FXS: A bit = 1, B bit = 1

FXO Originates

Step 1: FXO sets the B bit to 0. The B bit toggles with the ring generation:

To FXS: A bit = 0, B bit = 1

From FXS: A bit = 1, B bit = 1

Loop-Start Testing

How to test the signaling states of a loop-start trunk is discussed with reference to two viewpoints: from the demarc looking toward the CO and from the demarc looking toward the PBX.

Idle Condition (on-hook, initial state)

The idle condition is represented in Figure 14. The bridging clips are removed to isolate the CO from the PBX.

Looking toward the PBX, an open condition is observed between the T-R leads at the demarc.

Looking toward the CO from the demarc, ground is observed on the T lead and –48V is observed on the R lead. A voltmeter connected between T and R on the CO side of the demarc ideally reads close to –48V.

Figure 14 Outgoing (off-hook)

In order to test the operation toward the CO, remove the bridging clips and attach a test telephone set across the T-R leads toward the CO. The test set provides loop closure. The CO detects the loop closure, attaches a digit receiver to the circuit, establishes an audio path, and transmits dial tone toward the PBX. (See Figure 15.)

Figure 15

Once a dial tone is received by the test telephone, you can proceed to dial with either DTMF or dial-pulse signaling as allowed by the CO. Some COs are equipped to receive only dial-pulse addressing. Those equipped to receive DTMF can also receive dial pulse. When the first dialed digit is received, the CO removes dial tone.

After all digits have been dialed, the digit receiver is removed at the CO, and the call is routed to the distant station or switch. The audio path is extended over the outgoing facility, and audible call-progress tones are returned to the test telephone. Once the call is answered, voice signals can be heard over the audio path.

Incoming (ringing at destination)

A test telephone at the demarc can also be used to test loop-start trunks for incoming call operation. The test setup is the same as for outgoing calls. Typically the PBX technician calls a CO technician on another line and asks the CO technician to call the PBX on the trunk under test. The CO applies ringing voltage to the trunk. Ideally, the test phone at the demarc rings. The PBX technician answers the call on the test phone. If the technicians can talk to each other over the trunk under test, the trunk functions normally.

Tests between the PBX and the demarc with bridging clips removed are difficult. The loop-start interface circuits in most PBXs require battery voltage from the CO for their operation. If the voltage is not present, the trunk cannot be selected for outgoing calls. The usual procedure is to test the trunk from the demarc to the CO, first with the bridging clips removed as described, and then after installing the bridging clips. If the trunk fails to function properly when connected to the PBX, the problem is probably in the PBX or in the wiring between the PBX and the demarc. How Telephones Work

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Although most of us take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. If you want to talk to someone, all you have to do is pick up the phone and dial a few digits. You are instantly connected to that person, and you can have a two-way conversation. The telephone network extends worldwide, so you can reach nearly anyone on the planet. When you compare that to the state of the world just 100 years ago, when it might have taken several weeks to get a one-way written message to someone, you realize just how amazing the telephone is!

This illustration shows the entire telephone network, including a home connection, cell phone towers, long distance exchanges and transcontinental connections.

Now, we will look at the telephone device that you have in your house as well as the telephone network it connects to so you can make and receive calls.

The TelephoneSurprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the 1920s, you could connect it to the wall jack in your house and it would work fine!

The very simplest working telephone would look like this inside: As you can see, it only contains three parts and they are all simple:

 A switch to connect and disconnect the phone from the network - This switch is generally called the hook switch. It connects when you lift the handset.  A speaker - This is generally a little 50-cent, 8-ohm speaker of some sort.

 A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from your voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone.

That's it! You can dial this simple phone by rapidly tapping the hook switch -- all telephone switches still recognize "pulse dialing." If you pick the phone up and rapidly tap the switch hook four times, the phone company's switch will understand that you have dialed a "4."

The only problem with the phone shown above is that when you talk, you will hear your voice through the speaker. Most people find that annoying, so any "real" phone contains a device called a duplex coil or something functionally equivalent to block the sound of your own voice from reaching your ear. A modern telephone also includes a bell so it can ring and a touch-tone keypad and frequency generator. A "real" phone looks like this: Still, it's pretty simple. In a modern phone there is an electronic microphone, amplifier and circuit to replace the carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to generate a pleasant . But a regular $6.95 telephone remains one of the simplest devices ever.

The Telephone NetworkThe telephone network starts in your house. A pair of copper wires runs from a box at the road to a box (often called an entrance bridge) at your house. From there, the pair of wires is connected to each phone jack in your house (usually using red and green wires). If your house has two phone lines, then two separate pairs of copper wires run from the road to your house. The second pair is usually colored yellow and black inside your house. (See this Question of the Day for a description of the telephone boxes and wires that you see by the road.) A typical phone company box that you see by the side of the road. Click here to learn more.

Along the road runs a thick cable packed with 100 or more copper pairs. Depending on where you are located, this thick cable will run directly to the phone company's switch in your area or it will run to a box about the size of a refrigerator that acts as a digital concentrator.

This illustration shows the entire telephone network, including a home connection, cell phone towers, long distance exchanges and transcontinental connections. Click here to see the animated version!

The concentrator digitizes your voice at a sample rate of 8,000 samples per second and 8-bit resolution (see How Analog and Digital Recording Works for information on digitizing sounds). It then combines your voice with dozens of others and sends them all down a single wire (usually a coax cable or a fiber-optic cable) to the phone company office. Either way, your line connects into a line card at the switch so you can hear the dial tone when you pick up your phone.

If you are calling someone connected to the same office, then the switch simply creates a loop between your phone and the phone of the person you called. If it's a long-distance call, then your voice is digitized and combined with millions of other voices on the long-distance network. Your voice normally travels over a fiber-optic line to the office of the receiving party, but it may also be transmitted by satellite or by microwave towers. (See this Question of the Day for a more detailed description of long-distance calling.)

Creating Your Own Telephone NetworkNot only is a telephone a simple device, but the connection between you and the phone company is even simpler. In fact, you can easily create your own system using two telephones, a 9-volt battery (or some other simple power supply) and a 300-ohm resistor that you can get for a dollar at Radio Shack. You can wire it up like this: Your connection to the phone company consists of two copper wires. Usually they are red and green. The green wire is common, and the red wire supplies your phone with 6 to 12 volts DC at about 30 milliamps. If you think about a simple carbon granule microphone, all it is doing is modulating that current (letting more or less current through depending on how the sound waves compress and relax the granules), and the speaker at the other end "plays" that modulated signal. That's all there is to it!

The easiest way to wire up a private intercom like this is to go to a hardware or discount store and buy a 100-foot phone cord. Cut it, strip the wires and hook in the battery and resistor as shown. (Most cheap phone cords contain only two wires, but if the one you buy happens to have four, then use the center two.) When two people pick up the phones together, they can talk to each other just fine. This sort of arrangement will work at distances of up to several miles apart.

The only thing your little intercom cannot do is ring the phone to tell the person at the other end to pick up. The "ring" signal is a 90-volt AC wave at 20 hertz (Hz).

Hand Generated! You know the hand crank on those old- fashioned telephones? It was used to generate the ring-signal AC wave and sound the bell at the other end!

Calling SomeoneIf you go back to the days of the manual switchboard, it is easy to understand how the larger phone system works. In the days of the manual switchboard, there was a pair of copper wires running from every house to a central office in the middle of town. The sat in front of a board with one jack for every pair of wires entering the office.

Above each jack was a small light. A large battery supplied current through a resistor to each wire pair (in the same way you saw in the previous section). When someone picked up the handset on his or her telephone, the hook switch would complete the circuit and let current flow through wires between the house and the office. This would light the light bulb above that person's jack on the switchboard. The operator would connect his/her headset into that jack and ask who the person would like to talk to. The operator would then send a ring signal to the receiving party and wait for the party to pick up the phone. Once the receiving party picked up, the operator would connect the two people together in exactly the same way the simple intercom on the previous page was connected! It is that simple! In a modern phone system, the operator has been replaced by an electronic switch. When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know that the switch and your phone are working. The dial tone sound is simply a combination of 350-hertz tone and a 440-hertz tone, and it sounds like this:

 Click here to hear a dial tone.

(For more information on tones, see How Guitars Work.)

You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones, as shown here:

1,209 1,336 1,477 Hz Hz Hz 697 1 2 3 Hz 770 4 5 6 Hz 852 7 8 9 Hz 941 * 0 # Hz

A typical number that you dial sounds like this:

 Click here to hear a touch-tone number.

If the number is busy, you hear a busy signal that is made up of a 480-hertz and a 620-hertz tone, with a cycle of one-half second on and one-half second off, like this:

 Click here to hear a busy signal.

Telephone BandwidthIn order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3,000 hertz. All of the frequencies in your voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound. Compare these two voices:

 Click here to hear a normal voice.  Click here to hear the same voice on the telephone.

You can prove that this sort of filtering actually happens by using the following sound files:

 1,000-hertz tone  2,000-hertz tone

 3,000-hertz tone

 4,000-hertz tone

 5,000-hertz tone

 6,000-hertz tone

Call up someone you know and play the 1,000-hertz sound file on your computer. The person will be able to hear the tone clearly. The person will also be able to hear the 2,000- and 3,000- hertz tones. However, the person will have trouble hearing the 4,000-hertz tone, and will not hear the 5,000- or 6,000-hertz tones at all! That's because the phone company clips them off completely.

For lots more information on telephones, telephone networks and related technologies, check out the links on the next page. How Speakers Work

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In any sound system, ultimate quality depends on the speakers. The best recording, encoded on the most advanced storage device and played by a top-of-the-line deck and amplifier, will sound awful if the system is hooked up to poor speakers. A system's speaker is the component that takes the electronic signal stored on things like CDs, tapes and DVDs and turns it back into actual sound that we can hear.

A small speaker set for computer use

Now, we'll find out exactly how speakers do this. We'll also look at how speaker designs differ, and see how these differences affect sound quality. Speakers are amazing pieces of technology that have had a profound impact on our culture. But at their heart, they are remarkably simple devices.

Sound BasicsTo understand how speakers work, you first need to understand how sound works.

Inside your ear is a very thin piece of skin called the eardrum. When your eardrum vibrates, your brain interprets the vibrations as sound -- that's how you hear. Rapid changes in air pressure are the most common thing to vibrate your eardrum.

An object produces sound when it vibrates in air (sound can also travel through liquids and solids, but air is the when we listen to speakers). When something vibrates, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air as a traveling disturbance.

To see how this works, let's look at a simple vibrating object -- a bell. When you ring a bell, the metal vibrates -- flexes in and out -- rapidly. When it flexes out on one side, it pushes out on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them and so on. When the bell flexes away, it pulls in on these surrounding air particles, creating a drop in pressure that pulls in on more surrounding air particles, which creates another drop in pressure that pulls in particles that are even farther out and so on. This decreasing of pressure is called rarefaction.

In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. When the fluctuation wave reaches your ear, it vibrates the eardrum back and forth. Our brain interprets this motion as sound. We hear different sounds from different vibrating objects because of variations in:

 Sound-wave frequency - A higher wave frequency simply means that the air pressure fluctuates faster. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower.  Air-pressure level - This is the wave's amplitude, which determines how loud the sound is. Sound waves with greater amplitudes move our ear drums more, and we register this sensation as a higher volume.

A microphone works something like our ears. It has a diaphragm that is vibrated by sound waves in an area. The signal from a microphone gets encoded on a tape or CD as an electrical signal. When you play this signal back on your stereo, the amplifier sends it to the speaker, which re-interprets it into physical vibrations. Good speakers are optimized to produce extremely accurate fluctuations in air pressure, just like the ones originally picked up by the microphone. In the next section, we'll see how the speaker accomplishes this.

Making SoundIn the last section, we saw that sound travels in waves of air pressure fluctuation, and that we hear sounds differently depending on the frequency and amplitude of these waves. We also learned that microphones translate sound waves into electrical signals, which can be encoded onto CDs, tapes, LPs, etc. Players convert this stored information back into an electric current for use in the stereo system.

A speaker is essentially the final translation machine -- the reverse of the microphone. It takes the electrical signal and translates it back into physical vibrations to create sound waves. When everything is working as it should, the speaker produces nearly the same vibrations that the microphone originally recorded and encoded on a tape, CD, LP, etc. Traditional speakers do this with one or more drivers. A driver produces sound waves by rapidly vibrating a flexible cone, or diaphragm.

 The cone, usually made of paper, plastic or metal, is attached on the wide end to the suspension.  The suspension, or surround, is a rim of flexible material that allows the cone to move, and is attached to the driver's metal frame, called the basket.

 The narrow end of the cone is connected to the voice coil.

 The coil is attached to the basket by the spider, a ring of flexible material. The spider holds the coil in position, but allows it to move freely back and forth.

Some drivers have a dome instead of a cone. A dome is just a diaphragm that extends out instead of tapering in. A typical speaker driver, with a metal basket, heavy permanent magnet and paper diaphragm

The voice coil is a basic electromagnet. If you've read How Electromagnets Work, then you know that an electromagnet is a coil of wire, usually wrapped around a piece of magnetic metal, such as iron. Running electrical current through the wire creates a magnetic field around the coil, magnetizing the metal it is wrapped around. The field acts just like the magnetic field around a permanent magnet: It has a polar orientation -- a "north" end and and a "south" end -- and it is attracted to iron objects. But unlike a permanent magnet, in an electromagnet you can alter the orientation of the poles. If you reverse the flow of the current, the north and south ends of the electromagnet switch.

This is exactly what a stereo signal does -- it constantly reverses the flow of electricity. If you've ever hooked up a stereo system, then you know that there are two output wires for each speaker -- typically a black one and a red one.

The wire that runs through the speaker system connects to two hook-up jacks on the driver.

Essentially, the amplifier is constantly switching the electrical signal, fluctuating between a positive charge and a negative charge on the red wire. Since electrons always flow in the same direction between positively charged particles and negatively charged particles, the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second.

So how does this fluctuation make the speaker coil move back and forth? The electromagnet is positioned in a constant magnetic field created by a permanent magnet. These two magnets -- the electromagnet and the permanent magnet -- interact with each other as any two magnets do. The positive end of the electromagnet is attracted to the negative pole of the permanent magnetic field, and the negative pole of the electromagnet is repelled by the permanent magnet's negative pole. When the electromagnet's polar orientation switches, so does the direction of repulsion and attraction. In this way, the alternating current constantly reverses the magnetic forces between the voice coil and the permanent magnet. This pushes the coil back and forth rapidly, like a piston.

When the electrical current flowing through the voice coil changes direction, the coil's polar orientation reverses. This changes the magnetic forces between the voice coil and the permanent magnet, moving the coil and attached diaphragm back and forth.

When the coil moves, it pushes and pulls on the speaker cone. This vibrates the air in front of the speaker, creating sound waves. The electrical audio signal can also be interpreted as a wave. The frequency and amplitude of this wave, which represents the original sound wave, dictates the rate and distance that the voice coil moves. This, in turn, determines the frequency and amplitude of the sound waves produced by the diaphragm.

Different driver sizes are better suited for certain frequency ranges. For this reason, loudspeaker units typically divide a wide frequency range among multiple drivers. In the next section, we'll find out how speakers divide up the frequency range, and we'll look at the main driver types used in loudspeakers.

Chunks of the Frequency RangeIn the last section, we saw that traditional speakers produce sound by pushing and pulling an electromagnet attached to a flexible cone. Although drivers are all based on the same concept, there is a wide range in driver size and power. The basic driver types are:

 Woofers  Tweeters

 Midrange

Woofer

Tweeter

Midrange

Woofers are the biggest drivers, and are designed to produce low frequency sounds. Tweeters are much smaller units, designed to produce the highest frequencies. Midrange speakers produce a range of frequencies in the middle of the sound spectrum.

And if you think about it, this makes perfect sense. To create higher frequency waves -- waves in which the points of high pressure and low pressure are closer together -- the driver diaphragm must vibrate more quickly. This is harder to do with a large cone because of the mass of the cone. Conversely, it's harder to get a small driver to vibrate slowly enough to produce sounds. It's more suited to rapid movement.

To produce quality sound over a wide frequency range more effectively, you can break the entire range into smaller chunks that are handled by specialized drivers. Quality loudspeakers will typically have a woofer, a tweeter and sometimes a midrange driver, all included in one enclosure.

Of course, to dedicate each driver to a particular frequency range, the speaker system first needs to break the audio signal into different pieces -- low frequency, high frequency and sometimes mid-range frequencies. This is the job of the speaker crossover.

The most common type of crossover is passive, meaning it doesn't need an external power source because it is activated by the audio signal passing through it. This sort of crossover uses , capacitors and sometimes other circuitry components. Capacitors and inductors only become good conductors under certain conditions. A crossover capacitor will conduct the current very well when the frequency exceeds a certain level, but will conduct poorly when the frequency is below that level. A crossover acts in the reverse manner -- it is only a good conductor when the frequency is below a certain level. The typical crossover unit from a loudspeaker: The frequency is divided up by inductors and capacitors and then sent on to the woofer, tweeter and mid-range driver.

When the electrical audio signal travels through the speaker wire to the speaker, it passes through the crossover units for each driver. To flow to the tweeter, the current will have to pass through a capacitor. So for the most part, the high frequency part of the signal will flow on to the tweeter voice coil. To flow to the woofer, the current passes through an inductor, so the driver will mainly respond to low frequencies. A crossover for the mid-range driver will conduct the current through a capacitor and an inductor, to set an upper and lower cutoff point.

There are also active crossovers. Active crossovers are electronic devices that pick out the different frequency ranges in an audio signal before it goes on to the amplifier (you use an amplifier circuit for each driver). They have several advantages over passive crossovers, the main one being that you can easily adjust the frequency ranges. Passive crossover ranges are determined by the individual circuitry components -- to change them, you need to install new capacitors and inductors. Active crossovers aren't as widely used as passive crossovers, however, because the equipment is much more expensive and you need multiple amplifier outputs for your speakers.

Crossovers and drivers can be installed as separate components in a sound system, but most people end up buying speaker units that house the crossover and multiple drivers in one box. In the next section, we'll find out what these speaker enclosures do and how they affect the speaker's sound quality.

Boxes of SoundIn most loudspeaker systems, the drivers and the crossover are housed in some sort of speaker enclosure. These enclosures serve a number of functions. On their most basic level, they make it much easier to set up the speakers. Everything's in one unit and the drivers are kept in the right position, so they work together to produce the best sound. Enclosures are usually built with heavy wood or another solid material that will effectively absorb the driver's vibration. If you simply placed a driver on a table, the table would vibrate so much it would drown out a lot of the speaker's sound. Additionally, the speaker enclosure affects how sound is produced. When we looked at speaker drivers, we focused on how the vibrating diaphragm emitted sound waves in front of the cone. But, since the diaphragm is moving back and forth, it's actually producing sound waves behind the cone as well. Different enclosure types have different ways of handling these "backward" waves.

A typical sealed speaker enclosure that holds a tweeter, a woofer and a midrange driver.

The most common type of enclosure is the sealed enclosure, also called acoustic suspension enclosure. These enclosures are completely sealed, so no air can escape. This means the forward wave travels outward into the room, while the backward wave travels only into the box. Of course, since no air can escape, the internal air pressure is constantly changing -- when the driver moves in, the pressure is increased and when the driver moves out, it is decreased. Both movements create pressure differences between the air inside the box and the air outside the box. The air will always move to equalize pressure levels, so the driver is constantly being pushed toward its "resting" state -- the position at which internal and external air pressure are the same. In a sealed speaker setup, the driver diaphragm compresses air in the enclosure when it moves in and rarefies air when it moves out.

These enclosures are less efficient than other designs because the amplifier has to boost the electrical signal to overcome the force of air pressure. The force serves a valuable function, however -- it acts like a spring to keep the driver in the right position. This makes for tighter, more precise sound production.

Other enclosure designs redirect the inward pressure outward, using it to supplement the forward sound wave. The most common way to do this is to build a small port into the speaker. In these bass reflex speakers, the backward motion of the diaphragm pushes sound waves out of the port, boosting the overall sound level. The main advantage of bass reflex enclosures is efficiency. The power moving the driver is used to emit two sound waves rather than one. The disadvantage is that there is no air pressure difference to spring the driver back into place, so the sound production is not as precise. A bass reflex speaker produces two sound waves by moving one driver. When the driver compresses air forward, it rarefies it backward, and vice versa. The second sound wave is emitted from a port at the base of the speaker enclosure.

Passive radiator enclosures are very similar to bass reflex units, but in passive radiator enclosures, the backward wave moves an additional, passive driver, instead of escaping out of the port. The passive driver is just like the main, active drivers except it doesn't have an electromagnet voice coil, and it isn't connected to the amplifier. It is moved only by the sound waves coming from the active drivers. This type of enclosure is more efficient than sealed designs and more precise than bass reflex models.

Some enclosure designs have an active driver facing one way and a passive driver facing the other way. This dipole design diffuses the sound in all directions, making it a good choice for the rear channels in a home theater system. The backward air compression and rarefaction caused by the active driver push and pull on the passive driver. A speaker with a dipole design emits sound waves in both directions.

These are just a few of the many enclosure types available. There are a huge range of speaker units on the market, with a variety of unique structures and driver arrangements. Check out this page to learn about some of these designs.

Alternative Speaker DesignsMost loudspeakers produce sound with traditional drivers. But there are a few other technologies on the market. These designs have some advantages over traditional dynamic speakers, but they fall short in other areas. For this reason, they are often used in conjunction with driver units.

The most popular alternative is the electrostatic speaker. These speakers vibrate air with a large, thin, conductive diaphragm panel. This diaphragm panel is suspended between two stationary conductive panels that are charged with electrical current from a wall outlet. These panels create an electrical field with a positive end and a negative end. The audio signal runs a current through the suspended panel, rapidly switching between a positive charge and a negative charge. When the charge is positive, the panel is drawn toward the negative end of the field, and when the charge is negative, it moves toward the positive end in the field.

The diaphragm is alternately charged with a positive current and a negative current, based on the varying electrical audio signal. When the diaphragm is positively charged, it fluctuates toward the front plate, and when it is negatively charged it fluctuates toward the rear plate. In this way, it precisely reproduces the recorded pattern of air fluctuations.

In this way, the diaphragm rapidly vibrates the air in front of it. Because the panel has such a low mass, it responds very quickly and precisely to changes in the audio signal. This makes for clear, extremely accurate sound reproduction. The panel doesn't move a great distance, however, so it is not very effective at producing lower frequency sounds. For this reason, electrostatic speakers are often paired with a woofer that boosts the low frequency range. The other problem with electrostatic speakers is that they must be plugged into the wall and so are more difficult to place in a room.

Another alternative is the planar magnetic speaker. These units use a long, metal ribbon suspended between two magnetic panels. They basically work the same way as electrostatic speakers, except that the alternating positive and negative current moves the diaphragm in a magnetic field rather than an electric field. Like electrostatic speakers, they produce high-frequency sound with extraordinary precision, but low frequency sounds are less defined. For this reason, the planar magnetic speaker is usually used only as a tweeter. Both of these designs are becoming more popular with audio enthusiasts, but traditional dynamic drivers are still the most prevalent technology, far and away. You'll find them everywhere you go -- not only in stereo setups, but in alarm clocks, public address systems, televisions, computers, headphones and tons of other devices. It's amazing how such a simple concept has revolutionized the modern world! The telephone handset is defined1 as a "combination of a telephone transmitter and a telephone receiver mounted on a handle." The transmitter electrodes and the carbon-granule cup must be constructed so that the granules cannot fall away from the electrodes and open the circuit for any position in which the handset is held; that is, the transmitter must be non-positional.

Figure 2. Construction of typical telephone handset transmitters, (a. Courtesy Western Electric Co.; b. courtesy Kellogg Switchboard and Supply Co.)

Typical methods of construction are shown in Fig. 2. In each of these the diaphragm is formed and placed so that it acts as the front electrode. The other electrode is at the back of the carbon- granule cup or container. The cups are not filled entirely with granules because space must be left for expansion of the granules when the temperature rises. The diaphragm of Fig. 2(a) is cone shaped and ribbed so that it will be stiff and will move in and out somewhat like a piston. The diaphragm of Fig. 2(b) is damped acoustically so that it does not vibrate excessively at certain resonant frequencies. A second type of handset transmitter is shown in Fig. 3(a). The diaphragm consists of two thin aluminum-alloy cones. The two electrodes are separated by paper bellows. The frequency- response curve is shown in Fig. 3(b).

Figure 3. Construction (a) and frequency response (b) of a typical telephone handset transmitter. For the meaning of the word bar, see Fig. 9. (Courtesy Automatic Electric Co.) The telephone transmitters of Figs. 2 and 3 are known as "capsule types" because they are made as a unit and cannot be adjusted in the field. The characteristics of the handset telephone transmitters are superior to those of the transmitter of Fig. 1.