What is the history of clocks timeline? Information on the early non mechanical clocks in history and the changes on clocks to mechanical and electrical clocks.
CLOCK, a machine that indicates or records the time of day by dividing the earth’s period of rotation as accurately as possible into equal time intervals. Conventionally the period is divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. All clocks do this by means of some kind of regular motion that governs the indicating or recording elements so that they make equal movements in equal intervals of time.
EARLY NONMECHANICAL CLOCKS
The earliest timekeepers that have survived to the present are sundials and water clocks of ancient Egypt. A fragment of a simple sundial of about 1500 b. c. is in the Neues Museum in Berlin, and an alabaster water clock of about 1380 b. c. is in the Cairo Museum. The sundials were designed to divide the period from sunrise to sunset into 12 equal intervals, and the water clocks were used to divide the period from sunset to sunrise into 12 equal parts. So, in general, a day “hour” was not the same length as a night “hour,” and both varied according to the season.
The Egyptian water clock, also called a clepsydra, consisted of an alabaster bowl with sloping sides and a small hole in the bottom, from which a small metal outlet pipe protruded. (In surviving clepsydras the pipes have been destroyed by erosion.) It depended on the uniform flow of water through the hole to indicate passage of time. A plaster cast of a water clock of about 1380 b. c. is shown on this page. On the interior surface of the bowl there are 12 hour-scales, one for each month of the year. The vessel was filled with water to the topmost hour mark. The water was then allowed to leak out through the outlet pipe. Because of the shape of the bowl, the water level fell at an almost uniform rate.
Greece and Rome.
In the great Greek and Roman civilizations, sundials and water clocks continued to be the only timekeepers available. The sundials, though still made of stone, became rather more sophisticated in design and varied in form. One of the best and most common types was the so-called hemicycle of Berosus, which was invented about 300 b. c. The hollowed-out bowl was placed so that it faced due south. Lines on its inner surface corresponded to the 12 hours of the day from sunrise to sunset. A horizontal gnomen projected from the center of the top edge of the sundial so that the shadow of its tip fell on the hour scales.
Greek and Roman water clocks were generally cylindrical vessels into which water flowed at a uniform rate from a constant-head device. The time indicator was attached to a rising float. None of these water clocks has survived, but it is believed that some were quite elaborate and may have incorporated striking or alarm mechanisms.
A remarkable series of water clocks was constructed in China between about the 8th and 11th century a. d. These clocks were large structures with moving figures. They drove celestial globes and sounded the time by gongs or bells. Their timekeeping was controlled by the steady flow of water through a pipe, but the motion of the wheelwork was intermittent.
Sandglasses originated in Europe in the 14th century. They are essentially two glass bulbs, connected by a narrow neck, that contain a certain amount of sand. Intervals of time, usually small, are measured by the time it takes for all the sand to fall from the top bulb through the narrow neck to the bottom bulb. The sandglass is turned over for reuse.
Candle or oil-lamp clocks were also used to a very limited extent. The hour scales were marked on the side of the candle or on the glass oil reservoir. The shortening of the candle or the fall of the oil level indicated passage of time.
Sundials continued to be used in medieval Europe. Some were made portable and were equipped with a compass.
The first all-mechanical clocks known were probably made about 1300. They were large iron-framed structures, driven by weights. The cyclic motion they utilized was produced by an escapement known as the verge and foliot. A horizontal iron bar with adjustable weights at its ends (the foliot) was suspended at its center on a vertical spindle (the verge). The foliot was pushed first in one direction of rotation and then in the opposite direction by the teeth of the crown wheel, which “escaped,” one by one. The crown wheel itself was driven, through one or more stages of gearing, by a weight suspended from a rope that was coiled on a drum.
The function of these first European mechanical clocks was not to indicate the time on a dial, but to drive dials that gave astronomical indications, and to sound the hour. They were located in monasteries and public bell towers. The earliest surviving example is in Salisbury Cathedral, England. It was constructed in 1386.By this time, clocks for civil use in Europe were designed to divide the day-and-night period into 24 equal hours.
According to a contemporary manuscript, Richard of Wallingford made an elaborate clock, with many astronomical indications, for St. Albans Abbey in England in 1330. An equally elaborate clock was made by Giovanni Dondi in Italy. He completed it in 1364, and it survived for nearly 200 years. A reconstruction of it was made in England in 1961 and is now a permanent exhibit in the Smithsonian Institution in Washington, D. C.
Domestic clocks were scaled-down versions of the public clocks, but without the astronomical indications. Their frames and wheels were also of iron, and they usually had a striking mechanism or an alarm. In domestic clocks, the weighted bar controlling the timekeeping was soon replaced by a balance wheel, but the verge escapement was the same. The clock could then be regulated only by varying the driving weight. The earliest extant illustration of such a clock is in an illuminated manuscript of 1406 in the National Library of France in Paris.
From the mid-14th to the mid-17th century, the mechanism of weight-driven clocks underwent surprisingly little alteration in principle, but the construction of both the mechanism and the case became much more refined. After about 1600, brass took the place of iron for frames and wheels.
Springs were first used instead of weights for the driving power of clocks in the second half of the 15th century. They had the disadvantage that their drive was not constant; the pull, or torque, was greater when the spring was fully wound than when it was nearly unwound. Since the timekeeping j ability of the verge and foliot escapement depends very much on a constant driving force, some form of compensation was needed.
Early French spring-driven clocks employed a device called a fusee to equalize the force of the uncoiling spring. The spring is contained in a drum, and its power is transmitted through a gut cord or chain wound on the drum. The other end of the chain is wound round a spiral groove in a pulley, which then passes the drive on to the clock mechanism through gearing. When the spring is fully wound, the cord or chain unwinds from the narrow end of the pulley, where its radius and consequently its leverage are small; but as the spring runs down, the cord unwinds from increasingly wider parts of the pulley. Therefore the leverage increases, compensating for the weakening pull of the spring. Spring-driven clocks were often regulated by allowing the single crossbar of the balance wheel to strike two short pieces of gut that projected from an adjustable arm.
The earliest German spring-driven clocks employed a kind of friction brake called a stack-freed, which reduced the spring power markedly when it was fully wound, but only slightly toward the end of its run.
The earliest spring-driven clocks were in the form of metal drums a few inches in diameter, with the dial on the top. About the middle of the 16th century, clocks in the form of small vertical towers, with the dial on one side, took their place. They were often elaborately decorated.
The early domestic clocks had only a single hand, but in the 16th and early 17th centuries a few minute hands appeared. They were, however, hardly justified by the accuracy of the clocks, which were in error by at least a quarter of an hour a day.
Watches were first made in about 1500.
Great accuracy in time measurement was first made possible when the pendulum was applied as a regulator in clocks. Galileo, in 1582, had noticed that, as timed by his pulse, a swinging lamp in the cathedral of Pisa seemed to have the same time of swing for large as for small arcs. This observation was used in reverse by physicians, who timed their patients’ pulses by a simple pendulum, or “pulsilogium,” which they carried. Toward the end of his life, Galileo attempted to apply the pendulum to clocks as the timekeeping element. However, he died in 1642, before the clock he designed was constructed.
Christian Huygens, working independently in Holland, completed a preliminary model of a pendulum clock in 1656. In the following year, his clockmaker Salomon da Coster began constructing spring-driven pendulum clocks in The Hague. Two of these have survived to this day. The escapement was the same, but the pendulum, with its characteristic period, replaced the foliot balance or balance wheel. The employment of pendulums improved the timekeeping of clocks so much that all new clocks incorporated them, and many an older clock was converted to employ one.
Coster’s first pendulum clocks were housed in very simple wooden cases. In Holland and England these evolved into the typical wooden long-case, or grandfather, clock and the bracket clock. In France the evolution was to the mantel clock, which became very ornate in the 18th century.
An innovation in clocks parallel to that of the pendulum was the application of the balance spring to the balance wheel, which gave the regulating system a characteristic beat. Robert Hooke was the first to experiment with a straight spring, but it was again Huygens who, in 1675, first successfully applied a spring. He used a spiral spring in an arrangement that persists in clocks and watches to this day. Clocks employing spring-controlled balance wheels are regulated by varying the effective length of the spring.
Pendulums, used in conjunction with the verge escapement, made clocks accurate enough for normal domestic requirements, but for observatory or other scientific purposes considerably higher accuracy was required. Efforts were next directed toward improving the verge escapement mechanism, which gave an impulse to the pendulum on every swing and allowed the escape wheel to move through the pitch of one tooth for each double swing of the pendulum.
Huygens had discovered that the ideal curve of the pendulum swing for regular, or cyclic, motion is similar to, but not exactly the same as, the arc of a circle. He devised a mechanical means of changing the actual curve traced by the pendulum. However, other inventors found that the error introduced by allowing the pendulum to swing in a circular arc was more easily minimized by simply shortening the arc of swing. Attempts were also made to make the pendulum swing as free as possible from external interference. Pendulum clocks are usually regulated by raising or lowering the pendulum bob with a screw arrangement.
About 1670 the recoil, or anchor, escapement was devised by William Clement in England. It allowed the pendulum to swing with a smaller arc than the earlier verge allowed, and became the standard for domestic clocks. More accurate clocks were made possible by the deadbeat escapement, introduced by George Graham in England in about 1715. With the deadbeat escapement, the pendulum received an impulse near the center of its swing and was subject to only slight friction for the rest of its swing. A good pendulum clock with a deadbeat escapement is accurate to a few seconds per day.
Smaller portable clocks, whose timekeeping element is a balance and spring, require escapements of a different type, since in these clocks the balance may have quite a large arc of swing. For these, Graham devised his cylinder escapement in about 1720. It had properties similar to the deadbeat for pendulum clocks. Although it was difficult to make, it achieved considerable success and was widely used in watches and small clocks. It was later superseded by Thomas Mudge’s detached lever escapement, in which the balance wheel receives an impulse at the center of its swing and is entirely free of outside interference for the rest of its swing. It was invented in 1765 but was not in general use until much later. Since about the mid-19th century it has been almost universally employed in small clocks and watches.
Other types of escapements have been devised for use in large tower clocks, which have their hands exposed to the wind and precipitation. The varying forces on the hands lead, with an ordinary escapement, to a varying impulse to the pendulum. In a gravity escapement the force at the escape wheel is not applied directly to the pendulum, but lifts gravity arms, which, on their subsequent fall give a constant impulse to the pendulum. The first wholly successful escapement of this type was invented by Baron Grimthorpe in England and applied by him to Big Ben, the tower clock built for the new Houses of Parliament in 1854.
A pendulum rod expands or contracts with a rise or fall of temperature according to the material of which it is made. This change of length alters the time of swing, and for a brass rod will produce a losing rate of five seconds per day for a temperature rise of 10°F (5.5°C). The corresponding change in a steel rod is 2.5 seconds per day.
Devices compensating for the effects of temperature were invented by George Graham in London in 1721 and by John Harrison in the Lincolnshire village of Barrow-on-Humber in 1726. Graham attached to the lower end of the pendulum rod a jar containing mercury, which served as the pendulum bob. The upward expansion of the mercury compensated for the downward expansion of the rod as the temperature rose. Harrison’s gridiron pendulum was a more complicated device, made of brass and steel rods, which produced the same effect. These two types of compensation were used for all precision long-case clocks, in Britain and on the Continent, until the 19th century, when a combination of zinc and steel tubes was occasionally employed.
The problem was finally solved by Charles Guillaume, working in Paris in 1895, who produced a nickel-steel alloy called invar. Rods of invar effectively remain the same length over a wide temperature range. Methods used to compensate timekeepers controlled by a balance and spring involve the use of two metals fused together for the balance wheel, moving weights inwards or outwards to compensate for the change in stiffness of the balance spring with temperature.
In colonial days the domestic clocks found in America bore a close resemblance to their European counterparts, particularly the English long-case clock. After 1776, however, American clockmaking began to diverge into various new forms.
The “case on case” clock of about 1790 was an attempt to produce an apparent bracket clock (which is portable spring-driven clock) that was, in fact, driven by weights; and in 1802 Simon Willard patented his improved timepiecenow well-known as the banjo type—in which a heavy weight with limited fall acted on a small-diameter drum, to which was attached a large gear wheel. This developed later into the lyre and girandole types.
The next important American development was the mass production of wooden clock movements by Eli Terry, which led to the establishment of clockmaking as an industry in Connecticut. Terry introduced many of the well-known designs of shelf clocks that are now sought after by collectors. In the 1840’s Chaun-cey Jerome began the manufacture and export to Europe of mass-produced clocks with brass movements.
The train of gears that drives the striking mechanism of a clock is called the striking train. The striking train of early clocks was impelled by a separate driving weight, which was released each hour by the timekeeping mechanism. From the 14th to the late 17th century the striking train was of the count-wheel, or locking-plate, type. In this type, the number of blows struck is determined by a set of notches in the outer rim of a wheel, into which a pallet falls. With locking-plate striking, the hours must follow in their normal sequence.
In 1676, Edward Barlow, in England, introduced the rack, a striking mechanism in which a stepped cam, or snail, is attached to the axis of the hour hand. The number of blows struck is then determined by the depth of the step opposite the controlling pallet. In the rack type it is possible to utilize a repeating mechanism with which the nearest hour is struck “on request” as well as, or instead of, the automatic release at exactly the hour.
Electricity is used both to replace the springs and weights as a power source in clocks and to supply the regular motion necessary for timekeeping. It has enabled a single central master clock to control the dials of a number of remote clocks, and thus supply a unified timekeeping service to a whole building or institution. When used to give impulse to the pendulum, it greatly increases accuracy, because the impulse can be applied less frequently and with greater precision than in wholly mechanical clocks.
The earliest inventors of electric clocks were Alexander Bain in Scotland and Sir Charles Wheatstone in England, who worked independently and in some rivalry in 1840. Their master clocks sent a signal every second or two seconds to secondary clocks and thus controlled the time indicated on their dials. The system of synchronization that is most widely employed is that in which a master clock sends out a signal every minute or every half minute to advance the hands of remote clocks.
Other inventors between 1850 and 1890 tried systems in which each distant-controlled clock had a pendulum of its own, which was synchronized by signals from the master clock. Some incorporated systems of hourly correction.
In 1898, Robert James Rudd in England realized that it was possible to transfer the impulsing and counting functions of the escapement to a “slave” clock, which released an impulse to the “free” master pendulum every minute. The “slave” clock then received a synchronizing signal from the “free” master pendulum. Rudd built an all mechanical clock on this principle.
William Hamilton Shortt, in 1921, devised a clock similar in principle to Rudd’s, in which the linkage between the master pendulum and the slave clock is electrical. Shortt clocks have an accuracy of a few thousandths of a second per day. They were the standard timekeepers of the Greenwich Observatory from 1924 to 1942.
Synchronous Electric Clocks.
Wheatstone devised a system in 1869 in which the electric circuit was closed, and the pendulum acted as a dynamo. A more practical system of this type was developed by Henry E. Warren in 1916 in the United States. He used a small electric motor to drive the hands of a clock, through reduction gearing. It operated on commercial alternating current. Warren also developed a suitable master clock for use at power stations. The frequency of the power distributed could be controlled accordingly. The system was soon widely adopted in the United States and in Europe. Ordinary electric clocks are usually accurate to within a few seconds.
QUARTZ CRYSTAL AND ATOMIC CLOCKS
The Shortt free-pendulum clock appears to have reached the limit of accuracy that is attainable utilizing the pendulum for regular motion. Greater accuracy is obtainable by utilizing the vibrations of a quartz crystal. Quartz is a piezoelectric material. When it is compressed, small electric charges and voltages appear on its surface; these can be amplified and fed back to the crystal to maintain it in oscillation. The frequency of vibration of quartz is very high. It is on the order of 100,000 vibrations per second, but it can be reduced by various forms of electrical “gearing” to frequencies such as 60 vibrations per second. At this level it can drive a common synchronous electric clock.
Walter G. Cady, in 1922, was the first to employ a quartz crystal as a frequency standard, but it was W. A. Marrison who first worked out the full possibilities of quartz crystals for time-keeping. By 1929 he had constructed a ring-crystal timekeeper that was accurate to a hundredth of a second a day and 10 times as accurate for short periods of seconds or minutes. By 1938, L. Essen at the National Physical Laboratory in England had increased this accuracy by five times, which made the quartz clock more accurate than the Shortt clock. In 1942 the Greenwich Observatory replaced its Shortt clocks with quartz crystal clocks.
The most accurate clocks utilize the oscillations of atoms and molecules for regular motion. These extremely high-frequency oscillations are used to synchronize a lower-frequency quartz clock. Atomic clocks are accurate to one part in 1011 or in 1012. This is equivalent to an error of only one second in 3,000 years or in 30,000 years, respectively.