50 Scientific American William J. H. Andrewes is a museum consultant and maker of pre- cision sundials who has specialized in the history of time measure- ment for more than 35 years. He has worked at several scholarly institutions, including Harvard University. In addition to writing arti- cles for popular and academic journals, Andrewes edited The Quest for Longitude and co-wrote The Illustrated Longitude with Dava Sobel.

invention A Chronicle of Timekeeping Our conception of time depends on the way we measure it By William J. H. Andrewes

umankind’s efforts to tell time have helped drive the evolution of our technology and science throughout history. The need to gauge the divisions of the day and night led the ancient Egyptians, Greeks and Romans to create sundials, water and other early chronometric tools. Western Europeans adopted these tech­nologies, but by the 13th century, demand for a dependable timekeeping instrument led medieval ar- Htisans to invent the mechanical . Although this new device satisfied the requirements of monastic and urban communities, it was too inaccurate and unreliable for scientific application until the was employed to govern its operation. The precision timekeepers that were subsequently developed resolved the critical problem of finding a ship’s position at sea and went on to play key roles in the industrial revolution and the advance of Western civilization.

Today highly accurate timekeeping instru- began to measure time at least 5,000 years ago, in brief ments set the beat for most of our electronic de- introducing calendars to organize and coordi- vices. Nearly all computers, for example, contain nate communal activities and public events, to Devices for measuring time have been a quartz-crystal clock to regulate their opera- schedule the shipment of goods and, in particu- around for at least 5,000 years and tion. Moreover, not only do time signals beamed lar, to regulate cycles of planting and harvesting. today are essential for coordinating the operation of everything from cell down from Global Positioning System satellites They based their calendars on three natural cy- phones to power-distribution grids. calibrate the functions of precision navigation cles: the solar day, marked by the successive peri- The earliest mechanical clocks, in- equipment, they do so as well for cell phones, in- ods of light and darkness as the earth rotates on vented just before 1300, told time by stant stock-trading systems and nationwide its axis; the lunar month, following the phases of striking a bell. power-distribution grids. So integral have these the moon as it orbits the earth; and the solar By the early 1500s inventors had de- ) marks question a n ( digital time-based technologies become to our day-to- year, defined by the changing seasons that ac- veloped spring-driven mechanical a meric day lives that we recognize our dependency on company our planet’s revolution around the sun. clocks that were small enough to be them only when they fail to work. Before the invention of artificial light, the portable. Modern quartz and atomic clocks moon had greater social impact. And, for those have made it possible to keep extreme- Reckoning Dates living near the equator in particular, its waxing ly accurate time, which has opened ); scientific ); scientific face watch according to archaeological evidence, the Bab- and waning was more conspicuous than the the door to new applications. my ( my a

Al ylonians, Egyptians and other early civilizations passing of the seasons. Hence, the calendars

ScientificAmerican.com 51 Wheels and Springs Development of Early Mechanical Clockworks

Verge and Foliot Verge The innovative component of the first mechanical clocks (circa 1300) was the escape- ment, a device that both controlled the escape wheel’s rotation and transmitted the Cords power needed to sustain the motion of the oscillator, which in turn regulated the speed Pallet at which the timekeeper operated. The escape wheel, a sawtoothed crown, is driven by a Pallet gear train powered by a weighted cord wound around the axle. The clockwise rotation of the escape wheel is obstructed by two pallets protruding from a vertical shaft, called a Escape wheel verge, which carries a bar known as a foliot. When the top pallet checks the escape wheel’s rotation (causing a “tick”), Foliot the engaged wheel tooth gradually Verge forces the pallet back until it is free Cycloidal to escape. The wheel’s movement, cheeks however, is stopped almost immedi- Although Galileo and other 16th-cen- ately when the lower pallet arrests tury scientists knew about the poten- Weight another tooth (causing a “tock”) and tial of the pendulum as a timing in- strument, was the then pushes the verge in the oppo- Pallet site direction. Driven by the escape first to devise a pendulum clock. Huy- wheel, the to-and-fro oscillation of gens soon realized that a pendulum the verge and foliot continues until swinging in a small arc would perform Crutch the cord fully unwinds. The rate at Escape its oscillations faster than one moving which the mechanism operates can wheel in a large arc. He overcame this prob- be adjusted by moving the weights lem by installing two curved “cycloidal cheeks” (shown in side view) at the Rod on the foliot arms out (for slower) Pallet and in (for faster). pendulum’s suspension point. Acting on the suspension cords, these curved stops reduced the effective length of the pendulum as its arc increased so that it maintained a cycloidal rather The use of coiled springs as the motive force for timekeepers was made practical by than a circular path. Thus, in theory, the invention of the fusee in the early to mid-1400s. Although a spring is a compact the pendulum completed every swing power source, its force varies, increasing as it is wound more tightly. The fusee, a cone- in the same time period, regardless of shaped grooved pulley, was devised to compensate for the variable strength of a time- amplitude (swing distance). In Huy- Bob (swings keeper’s . The barrel, gens’s clock, the gravity-influenced perpendicular which houses the spring, is con- motion of the pendulum replaced the to the plane nected to the fusee by a cord or purely mechanically driven oscillation of the page) chain. When the mainspring is ful- of the horizontal foliot. Now it was the ly wound, the cord pulls on the Mainspring pendulum’s beat that regulated the narrow end of the fusee, where a action of the verge escapement and Barrel short torque arm produces rela- the rotation of the wheels, which in tively little leverage. As the clock turn delivered this far more reliable Fusee runs, the cord is gradually drawn and accurate time measurement to back onto the barrel. To compen- the hands of the clock dial. sate for the mainspring’s diminish- ing strength, the cord’s spiral track on the fusee increases in diameter. Cord Thus, the force delivered to the Anchor Escapement gear wheels of the timekeeper re- Developed around 1670 in England, the anchor escapement is a lever-based de- mains constant despite the chang- vice shaped like a ship’s anchor. The motion of a pendulum rocks the anchor so ing tension of its mainspring. that it catches and then releases each tooth of the escape wheel, which, in turn, provides an impulse to maintain the pendulum’s oscillation. Unlike the Anchor verge escapement used in early Spiral Balance Spring pendulum clocks, the anchor es- In 1675 Huygens invented the spiral capement permitted the pendulum balance spring. Just as gravity con- to travel in such a small arc that trols the swinging oscillation of a maintaining a cycloidal swing path pendulum in a clock, this spring became unnecessary. Moreover, regulates the rotary oscillation of a this invention made practical the balance wheel in portable timepiec- use of a long, seconds-beating pen- es. A balance wheel is a rotor that spins dulum and thus led to the de- one way and then the other, repeating the velopment of a new, floor- cycle over and over. Depicted here is a mod- Balance standing case design, which Spring ern version, finely balanced with adjustable wheel became known as the long- Escape timing screws. case, or grandfather, clock. wheel vid penney a d

52 Scientific American developed at the lower latitudes were in- land. That the Roman Catholic Church these fractions to be included on dials un- fluenced more by the lunar cycle than by should have played a major role in the in- til the 1660s, when the pendulum clock the solar year. In more northern climes, vention and development of clock technol- was developed. Minutes and seconds de- however, where seasonal agriculture was ogy is not surprising: the strict observance rive from the sexagesimal partitions of important, the solar year became more of prayer times by monastic orders occa- the degree introduced by Babylonian as- crucial. As the Roman Empire expanded sioned the need for a more reliable instru- tronomers. The word “minute” has its or- northward, it organized its calendar for ment of time measurement. Further, the igins in the Latin prima minuta, the first the most part around the solar year. To- Church not only controlled education but small division; “second” comes from se- day’s Gregorian calendar derives from also possessed the wherewithal to employ cunda minuta, the second small division. the Babylonian, Egyptian, Jewish and Ro- the most skillful craftsmen. Additionally, The sectioning of the day into 24 hours man calendars. the growth of urban mercantile popula- and of hours and minutes into 60 parts The Egyptians formulated a civil cal- tions in Europe during the second half of became so well established in Western endar having 12 months of 30 days, with the 13th century created demand for im- culture that all efforts to change this ar- five days added to approximate the solar proved timekeeping devices. By 1300 arti- rangement failed. The most notable at- year. Each period of 10 days was marked sans were building clocks for churches and tempt took place in revolutionary France by the appearance of special star groups cathedrals in France and Italy. Because the in the 1790s, when the government ad- (constellations) called decans. At the rise initial examples indicated the time by opted the decimal system. Although the of the star Sirius just before sunrise, striking a bell (thereby alerting the sur- French successfully introduced the me- which occurred around the all-impor- rounding community to its daily duties), ter, liter and other base-10 measures, the tant annual flooding of the Nile, 12 the name for this new machine was adopt- bid to break the day into 10 hours, each decans could be seen spanning the heav- ed from the Latin word for “bell,” clocca. consisting of 100 minutes split into 100 ens. The cosmic significance the Egyp- The revolutionary aspect of this new seconds, lasted only 16 months. tians placed in the 12 decans led them to timekeeper was neither the descending develop a system in which each interval weight that provided its motive force nor Portable Clocks of darkness (and later, each interval of the gear wheels (which had been around for centuries after the invention of the daylight) was divided into a dozen equal for at least 1,300 years) that transferred mechanical clock, the periodic tolling of parts. These periods became known as the power; it was the part called the es- the bell in the town church or clock tower temporal hours because their duration capement. This device controlled the was enough to demarcate the day for most varied according to the changing length wheels’ rotation and transmitted the pow- people. But by the 15th century a growing of days and nights with the passing of er required to maintain the motion of the number of clocks were being made for do- the seasons. Summer hours were long, oscillator, the part that regulated the speed mestic use. Those who could afford the lux- winter ones short; only at the spring and at which the timekeeper operated [for an ury of owning a clock found it convenient autumn equinoxes were the hours of explanation of early clockworks, see box to have one that could be moved from daylight and darkness equal. Temporal on opposite page]. The inventor of the place to place. Innovators accomplished hours, which were adopted by the Greeks clock escapement is unknown. portability by replacing the weight with a and then the Romans (who spread them coiled spring. The tension of a spring, how- throughout Europe), remained in use for Uniform Hours ever, is greater after it is wound. The con- more than 2,500 years. although the mechanical clock could be trivance that overcame this problem, Ingenious inventors devised sundials, adjusted to maintain temporal hours, it known as a fusee (from fusus, the Latin which indicate time by the length or direc- was naturally suited to keeping equal ones. term for “spindle”), was invented by an un- tion of the sun’s shadow, to track temporal With uniform hours, however, arose the known mechanical genius probably be- hours during the day. The sundial’s noctur- question of when to begin counting them, tween 1400 and 1450 [see middle left illus- nal counterpart, the water clock, was de- and so, in the early 14th century, a num- tration in box on opposite page]. This cone- signed to measure temporal hours at night. ber of systems evolved. The schemes that shaped device was connected by a cord to One of the first water clocks was a basin divided the day into 24 equal parts varied the barrel housing the spring: when the with a small hole near the bottom through according to the start of the count: Italian clock was wound, drawing the cord from which the water dripped out. The falling hours began at sunset, Babylonian hours the barrel onto the fusee, the diminishing water level denoted the passing hour as it at sunrise, astronomical hours at midday diameter of the spiral of the fusee compen- dipped below hour lines inscribed on the and “great clock” hours (used for some sated for the increasing pull of the spring. inner surface. Although these devices per- large public clocks in Germany) at mid- Thus, the fusee equalized the force of the formed satisfactorily around the Mediter- night. Eventually these and competing spring on the wheels of the timekeeper. ranean, they could not always be depend- systems were superseded by “small clock,” The importance of the fusee should ed on in the cloudy and often freezing or French, hours, which split the day, as not be underestimated: it made possible weather of northern Europe. we currently do, into two 12-hour periods the development of the portable clock as commencing at midnight. well as the subsequent evolution of the The Pulse of Time During the 1580s clockmakers received pocket watch. Many high-grade, spring- the earliest recorded weight-driven me- commissions for timekeepers showing driven timepieces, such as marine chro- chanical clock was installed in 1283 at minutes and seconds, but their mecha- nometers, continued to incorporate this Dunstable Priory in Bedfordshire, Eng- nisms were insufficiently accurate for device until after World War II.

ScientificAmerican.com 53 Innovative Clockworks from swing to swing was impossible, Huy- ance spring. Just as gravity controls the in the 16th century Danish astronomer gens devised a pendulum suspension that swinging oscillation of a pendulum in Tycho Brahe and his contemporaries tried caused the bob to move in a cycloid- clocks, this spring regulates the rotary os- to use clocks for scientific purposes, yet shaped arc rather than a circular one. In cillation of a balance wheel in portable even the best ones were still too unreliable. theory, this enabled it to oscillate in the timepieces. A balance wheel is a finely bal- Astronomers in particular needed a better same time regardless of its amplitude [see anced disk that rotates fully one way and tool for timing the transit of stars and top right illustration in box on page 52]. then the other, repeating the cycle over thereby creating more accurate maps of Pendulum clocks were about 100 times as and over [see bottom left illustration in the heavens. The pendulum proved to be accurate as their predecessors, reducing a box on page 52]. The spiral balance spring the key to boosting the accuracy and de- typical gain or loss of 15 minutes a day to revolutionized the accuracy of watches, pendability of timekeepers. Galileo, the about a minute a week. News of the inven- enabling them to keep time to within a Italian physicist and astronomer, and oth- tion spread rapidly, and by 1660 English minute a day. This advance sparked an al- ers before him experimented with pendu- and French artisans were developing their most immediate rise in the market for lums, but a 27-year-old Dutch astronomer own versions of this new timekeeper. watches, which were now no longer typi- and mathematician named Christiaan The advent of the pendulum not only cally worn on a chain around the neck but Huygens devised the first pendulum clock heightened demand for clocks but also re- were carried in a pocket. It also increased on Christmas Day in 1656. Huygens recog- sulted in their development as furniture. the demand for portable sundials by nized the commercial as well as the scien- National styles soon began to emerge: which watches could be set to time. tific significance of his invention immedi- English makers designed the case to fit At about the same time, Huygens heard ately, and within six months a local maker around the clock movement; in contrast, of an important English invention. The an- in the Hague had been granted a license to the French placed greater emphasis on the chor escapement, unlike the verge escape- manufacture pendulum clocks. shape and decoration of the case. Huy­­ ment he had been using in his pendulum Huygens saw that a pendulum travers- g­ens, however, had little interest in these clocks, allowed the pendulum to swing in ing a circular arc completed small oscilla- fashions, devoting much of his time to im- such a small arc that maintaining a cycloi- tions faster than large ones. Therefore, proving the device both for astronomical dal pathway became unnecessary. More- any variation in the extent of the pendu- use and for solving the problem of finding over, this escapement made practical the lum’s swing would cause the clock to gain longitude at sea. use of a long, seconds-beating pendulum or lose time. Realizing that maintaining a In 1675 Huygens devised another fun- and thus led to the development of a new constant amplitude (amount of travel) damental improvement, the spiral bal- case design. The longcase clock, commonly

Precision Timekeepers Two Modern Clocks Quartz Movement Cesium Fountain By the end of the 1960s watchmakers had taken a step away from (Atomic) Clock the traditional oscillating balance wheel with the development of Cesium fountain clocks derive their tim- 2 an electronic transistor-based oscillator comprising a tiny tuning ing reference from the frequency of an Cesium atoms fork whose vibrations were converted into the movement of the electron spin-flip transition that occurs hands. With the simultaneous rise of cheap, low-power integrated in a cesium 133 atom when probed by Microwave circuits and light-emitting diodes (LEDs), the search for a more ac- tuned microwaves. In a vacuum chamber, cavity curate timing element was on. Watchmakers soon adopted the six lasers slow the movements of gaseous 3 quartz-crystal resonator from radio transmitters. Quartz crystals cesium atoms, forming a small cloud 1 . Probe Detector are piezoelectric: they vibrate when subjected to a changing elec- A change in the operating frequency of laser tric voltage, and vice versa. When driven by a voltage at its har- the upper and lower lasers launches the monic frequency, the crystal oscillates resonantly, ringing like a atomic cloud, fountainlike 2 , up through bell. The output of the oscillator is then converted to pulses suit- a magnetically shielded microwave cavi- 4 able for the watch’s digital circuits, which operate an LED display ty 3 . As gravity pulls the cloud back or electrically actuated hands. down through the cavity, the electrons in Excited atoms the atoms interact with the microwaves for a second time. The microwaves flip the spins of the electrons, changing their Battery cell Quartz-crystal quantum-mechanical energy states. Af- controller Digital ter the cloud falls farther, a probe laser display causes the cesium to fluoresce, revealing Cesium whether its electrons have flipped their atoms spins, a reaction that is monitored by a

1 ) ( right a niels detector 4 . The detector’s output signal a n d

is then used to make the slight correction a l needed to tune the microwave emitter to a precise resonant frequency that can Microchip Lasers serve as the time beat for a clock. ); ( left vid penney a d

54 Scientific American known since 1876 as the whose longitude could be accurately ascer- refined that its fundamental design never (after a song by American Henry Clay tained using proved land-based methods. needed to be changed. Work), began to emerge as one of the most The great reward attracted a deluge of popular English styles. Longcase clocks harebrained schemes. Hence, the Board of Mass-Produced Timepieces with anchor and long pendu- Longitude, the committee appointed to re- at the turn of the 19th century, clocks lums can keep time to within a few seconds view promising ideas, held no meetings and watches were relatively accurate, but a week. The celebrated English clockmak- for more than 20 years. Two approaches, they remained expensive. Recognizing the er —and, subsequently, however, had long been known to be theo- potential market for a low-cost timekeep- his successor, George Graham—later mod­ retically sound. The first, called the lunar- er, two investors in Waterbury, Conn., took ified­ the anchor escapement to operate distance method, involved precise obser- action. In 1807 they gave Eli Terry, a clock- without recoil. This enhanced design, vations of the moon’s position in relation maker in nearby Plymouth, a three-year called the deadbeat escapement, became to the stars to determine the time at a ref- contract to manufacture 4,000 longcase the most widespread type used in preci- erence point from which longitude could clock movements from wood. A substantial sion timekeeping for the next 150 years. be measured; the other required a very ac- down payment made it possible for Terry to curate clock to make the same determina- devote the first year to fabricating machin- Solving the Longitude Problem tion. Because the earth rotates every 24 ery for mass production. By manufacturing when the royal observatory, Greenwich, hours, or 15 degrees in an hour, a two-hour interchangeable parts, he completed the was founded in England in 1675, part of its time difference represents a 30-degree dif- work within the terms of the contract. charter was to find “the so-much-desired ference in longitude. The seemingly over- A few years later Terry designed a longitude of places.” The first Astronomer whelming obstacles to keeping accurate wooden-movement shelf clock using the Royal, John Flamsteed, used clocks fitted time at sea—among them the often violent same volume-production techniques. Un- with anchor and deadbeat escapements to motions of ships, extreme changes in tem- like the longcase design, which required time the exact moments that stars crossed perature and variations in gravity at differ- the buyer to purchase a case separately, the celestial meridian, an imaginary line ent latitudes—led British physicist Isaac Terry’s shelf clock was completely self- that connects the poles of the celestial Newton and his followers to believe that contained. The customer needed only to sphere and defines the due-south point in the lunar-distance method, though prob- place it on a level shelf and wind it up. For the night sky. This allowed him to gather lematic, was the only viable solution. the relatively modest sum of $15, many more accurate information on star posi- Newton was wrong, however. In 1737 average people could now afford a clock. tions than had hitherto been possible by the board finally met for the first time to This achievement led to the establish- making angular measurements with sex- discuss the work of a most unlikely candi- ment of what was to become the renowned tants or quadrants alone. date, a Yorkshire carpenter named John Connecticut clockmaking industry. Although navigators could find their lat- Harrison. Harrison’s large and rather cum- Before the expansion of railroads in itude (their position north or south of the bersome longitude timekeeper­ had been the 19th century, towns in the U.S. and Eu- equator) at sea by measuring the altitude of used on a voyage to Lisbon and on the re- rope used the sun to determine local time. the sun or the polestar above the horizon, turn trip had proved its worth by correct- For example, because noon occurs in Bos- the heavens did not provide such a straight- ing the navigator’s dead reckoning of the ton about three minutes before it does in forward solution for finding longitude. ship’s longitude by 68 miles. Its maker, Worcester, Mass., Boston’s clocks were set Storms and currents often confounded at- however, was dissatisfied. Instead of ask- about three minutes ahead of those in tempts to keep track of distance and direc- ing the board for a West Indies trial, he re- Worcester. The expanding railroad net- tion traveled across oceans. The resulting quested and received financial support to work, however, needed a uniform time navigational errors cost seafaring nations construct an improved machine. standard for all the stations along the line. dearly, not only in prolonged voyages but After two years of work, still displeased Astronomical observatories began to dis- also in loss of lives, ships and cargo. with his second effort, Harrisonembarked ­ tribute the precise time to the railroad The severity of this predicament was on a third, laboring on it for 19 years. But companies by telegraph. The first public brought home to the British government by the time it was ready for testing, he re- time service, introduced in 1851, was based in 1707, when an admiral of the fleet and alized that his fourth marine timekeeper, on clock beats wired from the Harvard more than 1,600 sailors perished in the a five-inch-diameter watch he had been College Observatory in Cambridge, Mass. wrecks of four Royal Navy ships off the developing simultaneously, was better. On The Royal Observatory introduced its coast of the Scilly Isles. Thus, in 1714, a voyage to Jamaica in 1761, Harrison’s time service the next year, creating a sin- through an act of Parliament, Britain of- oversize watch performed well enough to gle standard time for Britain. fered substantial prizes for practical solu- win the prize, but the board refused to The U.S. established four time zones in tions to finding longitude at sea. The larg- give him his due without further proof. A 1883. By the next year the governments of est prize, £20,000 (roughly 200 times the second sea trial in 1764 confirmed his suc- all nations had recognized the benefits of annual wage of a skilled engineer of the cess. Harrison was reluctantly granted a worldwide standard of time for naviga- time), would be given to the inventor of an £10,000. Only when King George III in- tion and trade. At the 1884 International instrument that could determine a ship’s tervened in 1773 did he receive the re- Meridian Conference in Washington, D.C., longitude to within half a degree, or 30 maining prize money. Harrison’s break- the globe was divided into 24 time zones. nautical miles, when reckoned at the end through inspired further developments. Delegates chose the Royal Observatory as of a voyage to a port in the West Indies, By 1790 the marine chronometer was so the prime meridian (zero degrees lon­

ScientificAmerican.com 55 A Brief History of time Clocks That Revolutionized Timekeeping

Hemispherical Sundial Pendulum Clock H1 Sea Clock (Italy, circa A.D. 100) (the Netherlands, 1657) (England, 1735) The shadow’s direction on this ingenious Dutch scientist Christiaan Huygens finally solved the cen­turies- Roman sundial shows the time of day grant­ed Salomon Coster the right to old problem of finding longitude at sea in (just after the eighth hour of daylight), make the first pendulum clocks of his 1759 with his fourth marine timekeeper. while its length indicates the time of year design, revolutionizing the European This is an exact replica of his first sea (summer solstice). clockmaking industry. clock, called H1. gitude, the line from which all other lon- making business in Roxbury, Mass., to dis- tivity higher at the Wal­tham factory but gitudes are measured) in part because cuss mass-production methods for watch- production costs were less. Even some of two thirds of the world’s shipping already es. Howard and his partner gave Dennison the lower-grade American watches could used Greenwich time for navigation. space to experiment and develop machin- be expected to keep reasonably good time. ery for the project. By the fall of 1852, 20 The watch was at last a commodity acces- Watches for the Masses watches had been completed under Den- sible to the masses. many clockmakers of this era realized that nison’s supervision. His workmen fin- Because women had worn bracelet the market for watches would far exceed ished 100 watches by the following spring, watches in the 19th century, wristwatches that for clocks if production costs could and 1,000 more were produced a year lat- were long considered feminine accoutre- be reduced. The problem of mass-fabri- er. By that time the manufacturing facili- ments. During World War I, however, the cating interchangeable parts for watches, ties in Roxbury were proving too small, so pocket watch was modified so that it could however, was considerably more compli- the newly named Boston Watch Company be strapped to the wrist, where it could be cated because the precision demanded in moved to Walth­ am,­ Mass., where by the viewed more readily on the battlefield. making the necessary miniaturized com- end of 1854 it was assembling 36 watches With the help of a substantial marketing ponents was so much greater. Although a week. campaign, the masculine fashion for wrist­ improvements in quantity man­ufacture­ The American Waltham Watch Com- watches caught on after the war. Self-wind­ had been instituted in Europe since the pany, as it eventually became known, ben- ing mechanical wristwatches made their late 18th century, European watchmakers’ efited greatly from a huge demand for appearance during the 1920s. fears of saturating the market and threat- watches during the Civil War, when Union ening their workers’ jobs by abandoning Army forces used them to synchronize High-Precision Clocks traditional practices stifled most thoughts operations. Improvements in fabrication at the end of the 19th century, Sigmund of introducing machinery for the produc- techniques further boosted output and Riefler, based in Munich, developed a rad- tion of interchangeable watch parts. reduced prices significantly. Meanwhile ical new design of regulator—a highly ac- Disturbed that American watchmak- other U.S. companies formed in the hope curate timekeeper­ that served as a stan- ers seemed unable to compete with their of capturing part of the burgeoning trade. dard for controlling others. Housed in a counterparts in Europe, which controlled The Swiss, who had previously dominat- partial vacuum to minimize the effects of the market in the late 1840s, a watchmak- ed the industry, grew concerned when barometric pressure and equipped with a er in Maine named Aaron L. Dennison their exports plummeted in the 1870s. pendulum largely unaffected by tempera- met with Edward Howard, who had es- The investigator they sent to Massachu- ture variations, Riefler’s regulators at- tablished a successful clock- and scale- setts discovered that not only was produc- tained an accuracy of a tenth of a second a

56 Scientific American capable of measuring discrepancies in the earth’s spin, however, meant that a change was necessary. A new definition of the sec- ond, based on the resonant frequency of the cesium atom, was adopted as the new standard unit of time in 1967. The precise measurement of time is of such fundamental importance to science and technology that the search for ever greater accuracy continues. The perfor- mance of atomic clocks had been improv- ing by a factor of at least 10 per decade for about 50 years. But over the past decade improvements in atomic clock accuracy have dramatically accelerated. Recent ad- vances in laser science—particularly the Nobel Prize–winning development of fem­ to­sec­ond laser frequency combs—and atomic physics have enabled the develop- Glass-Tank Regulator ment of many new types of optical atomic (Germany, 1895) clocks, some based on transitions in single Shelf Clock (U.S., circa 1816) Sigmund Riefler made a significant improve- ions in electromagnetic traps and some Eli Terry designed a shelf clock with ment in timekeeping when he designed a based on collections of cold neutral atoms interchangeable parts, giving birth to the clock that operated in a partial vacuum. Com- held in lattices formed by laser light. Sev- Connecticut clockmaking industry, and bined with a pendulum made from a metal granted patent rights to Seth Thomas, who alloy with very low thermal expansion, this eral of these atomic clocks are already sta- made this example, one of the earliest Riefler regulator could keep time to a 10th ble to within a few hundred femtoseconds mass-produced clocks. of a second per day. per day and continue to rapidly improve. At this level of performance, formerly negligible effects become important and day and were thus adopted by nearly ev- in 1939 varied by only two thousandths of measurable. For example, the best atomic ery astronomical observatory. a second a day. By the end of World War clocks can now measure changes in gravity Further progress came several decades II, this accuracy had improved to the over the distance of a stair step, tiny mag- later, when English railroad engineer equivalent of a second every 30 years. netic fields generated by heart and brain William H. Shortt designed a so-called free Quartz-crystal technology did not re- activity, and other quantities such as tem- pendulum clock that reputedly kept time main the premier frequency standard for perature and acceleration. Companies are to within about a second a year. Shortt’s long either, however. By 1948 Harold Ly- now man­ufacturing “chip-scale” atomic system incorporated two pendulum clocks, ons and his associates at the National Bu- clocks the size of a quarter. In addition to one a “master” (housed in an evacuated reau of Standards in Washington, D.C., had keeping time with increasing accuracy, tank) and the other a “slave” (which con- based the first atomic clock on a far more new generations of atomic clocks will be tained the time dials). Every 30 seconds precise and stable source of timekeeping: used as exquisite sensors for myriad appli- the slave clock gave an electromagnetic im- an atom’s natural resonant frequency, the cations and will become ever smaller and pulse to, and was in turn regulated by, the periodic oscillation between two of its en- more portable. master clock pendulum, which was thus ergy states [see right illustration in box on Although our ability to measure time nearly free from mechanical disturbances. page 54]. Subsequent experiments in both will surely improve in the future, nothing Although Shortt clocks began to dis- the U.S. and England in the 1950s led to will change the fact that it is the one thing place Rieflers as observatory regulators the development of the cesium-beam of which we will never have enough. during the 1920s, their superiority was atomic clock. Today the averaged times of more to explore short-lived. In 1928 Warren A. Marrison, cesium clocks in various parts of the world an engineer at Bell Laboratories, then in provide the standard frequency for Coordi- Greenwich Time and the Discovery of the Longitude. New York City, discovered an extremely nated Universal Time, which has an accu- Derek Howse. Oxford University Press, 1980. History of the Hour: Clocks and Modern Temporal Or- uniform and reliable frequency source racy of better than one nanosecond a day. ders. Gerhard Dohrn-van Rossum. Translated by Thomas that was as revolutionary for timekeep- Up to the mid-20th century, the sidere- Dunlap. University of Chicago Press, 1996. ing as the pendulum had been 272 years al day, the period of the earth’s rotation on The Quest for Longitude: The Proceedings of the Longi­ earlier. Developed originally for use in ra- its axis in relation to the stars, was used to tude Symposium, Harvard University, Cambridge, Mas- dio broadcasting, the quartz crystal vi- determine standard time. This practice sachusetts, November 4–6, 1993. Edited by William J. H. Andrewes. Collection of Historical Scientific Instruments, brates at a highly regular rate when excit- had been retained even though it had Harvard University, 1996. ed by an electric current [see left illustra- been suspected since the late 18th century Time: From Earth Rotation to Atomic Physics. Dennis D. tion in box on page 54]. The first quartz that our planet’s axial rotation was not en- McCarthy and P. Kenneth Seidelmann. Wiley-VCH, 2009. itkin tep h en P itkin

S clocks installed at the Royal Observatory tirely constant. The rise of cesium clocks

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