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WIRELESS COURSE in Twenty Lessons by ^*C\&^?Ite^ *^

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WIRELESS COURSE in Twenty Lessons by ^*C\&^?Ite^ *^ University of California Berkeley WIRELESS COURSE in Twenty Lessons by ^*C\&^?ite^ *^ Published by TH ELECTRO IMPORTING CO NEW YORK Lesson Number One THE PRINCIPLES OF ELECTRICITY. ^iiN the study of wireless telegraphy, many electrical terms fl| and instruments are encountered, making it necessary for the beginners to obtain a working knowledge of elec- tricity before invading the more difficult subject of wireless. We have therefore devoted the first, second and third lessons to a concise and practical course in elementary electricity. We do not claim that it is complete, inasmuch as our course only covers electricity in general to give the student a better under- standing of wireless telegraphy. For a better knoivledge of electricity, we recommend the reader to the many excellent text books which cover the subject in a thorough manner. Electricity in its simplest form was known to the ancients many cen- turies before the Christian era. Thales, of Miletus, a city of Asia Minor, in the seventh century B. C., described the remarkable property of attraction and repulsion which amber possesses when rubbed. When being thus rubbed he found that it would attract particles of dust, dry leaves, straws, etc. This phenomenon was noted from time to time in the succeeding centuries, but it was not until 1600 A. D., that Dr. Gilbert of Colchester, England, took up the study of this subject. Because of the thoroughness with which he delved into the study of electricity, he is considered as the founder of the science of electricity. He gave the name of electricity to the peculiar force, which he derived from the Greek name "Elektron," meaning Amber. Electricity is found in two forms, in one it exists as a charge upon a body, and is known a? static electricity, while in the othes form it consists of a moving current through a wire, known as dynamic electricity. We there- fore have : Electrostatic electricity, that branch of the science which treats with electricity at rest. Electrodynamic electricity, that branch of the science which treats with electricity in motion. If a glass rod is rubbed with a silk handerchief and brought near a small pith ball, (made of dry flowers), which has been suspended by a silk thread, there will be an attraction of the pith ball towards the rod. However, as soon as the pith ball touches the rod, another action takes place: the ball Copyright 1912 by K. I. Co. 2 WIRELESS COURSE LESSON NO. 1 being repelled from the rod. The explanation is. that the ball originally held a charge opposite to that held by the rod, the charge being neutralized on touching the rod, and the surplus charge of the rod being carried on to the pith ball. Being that both the same charges exist on the ball and the rod, both will repel each other. Two kinds of electricity are produced by friction, the kind of charge being dependent on the substances rubbed together. Thus if glass is rubbed with silk it becomes charged with positive electricity. On the other hand, sealing wax receives a negative charge if rubbed witn flannel. Positive elec- tricity is represented by a plus sign (+),. and negative electricity by the minus sign ( -). Where the current has treen perfectly neutralized so that no polar- of ity exist:-,, a combination both signs is used ( + ). While a charge may be given to a body by contact, it is also possible to charge a body at a distance, and by what is known as induction. If an electrified rod is brought near a glass cylinder, the latter will receive a temporary charge which disappears again when the rod is removed from the vicinity of the cylinder. However, if 'a permanent charge is desired, the glass cylinder is touched by the hand while the rod is held in the other hand near the end opposite to that being touched. A body touched or grounded while near a charged body is electfified oppositely. A body brought near a charge of electricity is electrified oppositely on the near end and similarly on the far end The following table represents electrical conductors and non-conductors in their respective value : Conductors. Insulators (or non-conductors) Silver Dry air Copper Shellac Other metals Paraffin Ebonite Charcoal Amber India Rubber Plumbago Rosin Silk Damp Earth Sulphur Paper Water containing solids Glass Oils Moist air Mica It must be noted carefully that the conductors do not hold static charges on them, and are therefore known as "non-ele.ctrics." The insulators, which do not carry current, hold static charges and are known as "electrics". The capacity of a body in electricity denotes its ability to retain a charge. The total quantity that can be held depends directly on the (surface) capacity; but if we consider a certain amount of electricity, it will" charge a body of small capacity to a higher degree than it would one of a larger capac- on the ity, because it can spread out more on the surface of the. larger than smaller. One of the most familiar types of capacity is in the form of a glass jar or bottle, coated on its inside and outside with tinfoil, held on to the glass by first shellac or other adhesive. This is known as the Leyden jar, Fig. 1, the Fig. 1 Fig. 2 one having been produced at Leyden, Holland. A brass rod with a ball at its a chain end is passed through the wooden cover and makes contact either by WIRELESS COURSE LKSSUN NO. 1 sides of the tin foil. The outer foil be con- or a spring clip to the inner may the outside is nected by other means. To charge the Leyden jar, coating ball is touched with some grounded and the inside contact rod charged inner and outer are connected body. To discharge the jar, the coatings which consists of two connected together by means of a discharger, Fig 2, brass balls mounted on ah ebonite handle. inner and outer for By means of a Leyden jar having brass cups substitutes to the tin foil, it may be noticed that after the jar has been charged brass and these carefully removed, the charge will not be found in either is held the surface. It will also cup, proving that the charge really by glass there is one be noticed that in any Leyden jar, when it is discharged, large that the electric spark, and an instant after a weak spark, proving charge the first soaks into .the glass dielectric, and does not release itself upon discharge. If a heavy charge of current is to be stored, a number of Leyden jars are employed, all the inner coatings being connected together, and all the outsides connected together as shown in the illustration, Fig. 3. These may all be charged or discharged together. The capacity of a condenser depends on the size and shape of the plate and the distance between them, as well as the insulating medium (dielectric). The larger the plates, the greater the amount of current required to charge them, hence the greater the capacity. By decreasing the distance between the plates the capacity is also increased, since the nearer a body is brought r unde the influence of a charged body, the more electricity it will retain. Specific Inductive Capacity, is the name given to the ratio of the capacity of any condenser, for a given insulating material, to its capacity with air as a dielectric. The table below illustrates the relative dielectric value of various materials. As an example of -its use, if a condenser has a certain capacity with air as a dielectric, it will have 2.05 times that capacity if Petroleum is substituted for the air as the dielectric. Relative Value of Inductive Capacities. Glass 6.5 to 10 Paraffin 1.9 Shellac .2.9 to 3.7 Carbonic Acid 1.000 Sulphur 2.8 to 3.2 Air 1. Ebonite 2.7 Hydrogen 999 India Rubber 2.34 Vacuum .94 Petroleum 2.05 To produce static electricity in large amounts, a machine employing the principle of friction is often used for experimental purposes. There are various types of these frictional machines, the most popular type being a glass cylinder upon which a silk flap rubs as it is turned. The charge is gathered by appropriate collectors. The most successful machine of the type illustrated herewith, Fig 4 is the influence static machine, which consists of a number of plates upon which tin foil sectors have been placed These plates revolve in opposite directions, and the current is gathered by suitable collectors. The small machine shown in the cut generates sufficient current to give a spark 3 inches long, under all conditions, even on a rainy day. WIRELESS COURSE-LESSON NO. 1 Fig:. 4 CURRENT GALVANIC ELECTRICITY. In the foregoing pages we have only considered static electricity, which is not used extensively in commercial activities, inasmuch as it does not possess such useful characteristics as the current electricity. We have three kinds of current electricity, as follows : Continuous or direct current, is current which flows in one direction only Alternating current, is current which vflows in opposite directions chang- ing its direction periodically. Pulsating current, is current which flows in one direction, but is inter- rupted periodically. In explaining the properties^of current electricity and the meaning of its terms, a striking similarity between water and the electric current is made use of to serve as an effective example. We will therefore consider a tank of water several feet above the sur- rounding ground, as in the instance of reservoirs of municipal water supplies.
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