Harrison's breakthrough inspired further developments. By the marine chronometer was so refined that its fundamental design never needed to be changed. Recognizing the potential market for a low-cost timekeeper, two investors in Waterbury, Conn.
In they gave Eli Terry, a clockmaker in nearby Plymouth, a three-year contract to manufacture 4, longcase clock movements from wood. A substantial down payment made it possible for Terry to devote the first year to fabricating machinery for mass production. By manufacturing interchangeable parts, he completed the work within the terms of the contract. A few years later Terry designed a wooden-movement shelf clock using the same volume-production techniques.
Unlike the longcase design, which required the buyer to purchase a case separately, Terry's shelf clock was completely self-contained. The customer needed only to place it on a level shelf and wind it up. For the relatively modest sum of 15, many average people could now afford a clock. This achievement led to the establishment of what was to become the renowned Connecticut clockmaking industry. Before the expansion of railroads in the 19th century, towns in the U.
For example, because noon occurs in Boston about three minutes before it does in Worcester, Mass. The expanding railroad network, however, needed a uniform time standard for all the stations along the line. Astronomical observatories began to distribute the precise time to the railroad companies by telegraph. The first public time service, introduced in , was based on clock beats wired from the Harvard College Observatory in Cambridge, Mass.
The Royal Observatory introduced its time service the next year, creating a single standard time for Great Britain. The U. By the next year the governments of all nations had recognized the benefits of a worldwide standard of time for navigation and trade.
Signatories chose the Royal Observatory as the prime meridian zero degrees longitude, the line from which all other longitudes are measured in part because two thirds of the world's shipping already used Greenwich time for navigation. The problem of mass-fabricating interchangeable parts for watches, however, was considerably more complicated because the precision demanded in making the necessary miniaturized components was so much greater.
Although improvements in quantity manufacture had been instituted in Europe since the late 18th century, European watchmakers' fears of saturating the market and threatening their workers' jobs by abandoning traditional practices stifled most thoughts of introducing machinery for the production of interchangeable watch parts. Disturbed that American watchmakers seemed unable to compete with their counterparts in Europe, which controlled the market in the late s, a watchmaker in Maine named Aaron L.
Dennison met with Edward Howard, the operator of a clock factory in Roxbury, Mass. Howard and his partner gave Dennison space to experiment and develop machinery for the project.
By the fall of , 20 watches had been completed under Dennison's supervision. His workmen finished watches by the following spring, and 1, more were produced a year later. By that time the manufacturing facilities in Roxbury were proving too small, so the newly named Boston Watch Company moved to Waltham, Mass.
The American Waltham Watch Company, as it eventually became known, benefited greatly from a huge demand for watches during the Civil War, when Union Army forces used them to synchronize operations. Improvements in fabrication techniques further boosted output and cut prices. Meanwhile other U. The Swiss, who had previously dominated the industry, grew concerned when their exports plummeted in the s. The investigator they sent to Massachusetts discovered that not only was productivity higher at the Waltham factory but production costs were less.
Even some of the lower-grade American watches could be expected to keep reasonably good time. The watch was at last a commodity accessible to the masses. Because women had worn bracelet watches in the 19th century, wristwatches were long considered feminine accoutrements.
During World War I, however, the pocket watch was modified so that it could be strapped to the wrist, where it could be viewed more readily on the battlefield. With the help of a substantial marketing campaign, the masculine fashion for wristwatches caught on after the war.
Self-winding mechanical wristwatches made their appearance during the s. Housed in a partial vacuum to minimize the effects of barometric pressure and equipped with a pendulum largely unaffected by temperature variations, Riefler's regulators attained an accuracy of a tenth of a second a day and were thus adopted by nearly every astronomical observatory.
Further progress came several decades later, when English railroad engineer William H. Shortt designed a so-called free pendulum clock that reputedly kept time to within about a second a year. Shortt's system incorporated two pendulum clocks, one a master housed in an evacuated tank and the other a slave which contained the time dials. Every 30 seconds the slave clock gave an electromagnetic impulse to, and was in turn regulated by, the master clock pendulum, which was thus nearly free from mechanical disturbances.
Although Shortt clocks began to displace Rieflers as observatory regulators during the s, their superiority was short-lived. In Warren A. Marrison, an engineer at Bell Laboratories in New York, discovered an extremely uniform and reliable frequency source that was as revolutionary for timekeeping as the pendulum had been years earlier.
Developed originally for use in radio broadcasting, the quartz crystal vibrates at a highly regular rate when excited by an electric current [ see illustration in box on opposite page ].
The first quartz clocks installed at the Royal Observatory in varied by only two thousandths of a second a day. By the end of World War II, this accuracy had improved to the equivalent of a second every 30 years. Quartz-crystal technology did not remain the premier frequency standard for long either, however.
Subsequent experiments in both the U. Today the averaged times of cesium clocks in various parts of the world provide the standard frequency for Coordinated Universal Time, which has an accuracy of better than one nanosecond a day.
Up to the midth century, the sidereal day, the period of the earth's rotation on its axis in relation to the stars, was used to determine standard time.
This practice had been retained even though it had been suspected since the late 18th century that our planet's axial rotation was not entirely constant. The rise of cesium clocks capable of measuring discrepancies in the earth's spin, however, meant that a change was necessary. A new definition of the second, based on the resonant frequency of the cesium atom, was adopted as the new standard unit of time in The precise measurement of time is of such fundamental importance to science that the search for even greater accuracy continues.
Current and coming generations of atomic clocks, such as the hydrogen maser a frequency oscillator , the cesium fountain and, in particular, the optical clock both frequency discriminators , are expected to deliver an accuracy more precisely, a stability of femtoseconds quadrillionths of a second over a day [see Ultimate Clocks, by W. Wayt Gibbs, on page 56]. Although our ability to measure time will surely improve in the future, nothing will change the fact that it is the one thing of which we will never have enough.
Mechanical clocks differed by using an escapement mechanism to regulate time. The balance wheel on a watch or the pendulum on a grandfather's clock is an escapement -- a mechanism that ticks in a steady rhythm and lets the gears move forward at a steady rate in little equal jumps. The first escapement we know about was described in AD by the French engineer Villard de Honnecourt; but it wasn't used to control a clock.
Instead, it was used in a cute little gadget that steadily pointed at the sun while it moved through the daytime sky. Monastery records after , for the next hundred years, refer to clock bells, to gearing, to clock towers. But clock terminology rode right through the changeover.
The first clear drawing of a mechanical clock was given us by Jacopo di Dondi and his son in , and they'd probably been building them for at least 20 years by then. We can't be sure, but the first mechanical clock was probably made in the late s.
It's strange that such an earth-shattering change could be that invisible. Water clock inaccuracies had bottomed out at around 15 minutes a day, and that's about as well as the first mechanical clocks did. But now, suddenly, engineers began to cut that error in half every 30 years, right up into the 20th century.
The two major drawbacks to weights was that they were unavoidably large and that they could not be moved about, but improved power sources developed over generations led to the development of compact and portable timepieces.
A regulator is a mechanism that autonomously regulates the speed of rotation or other movement of a mechanical device. Early mechanical clocks used a type of regulator known as a foliot balance.
Later improvements in regulator mechanisms led to greater accuracy and portability of timepieces. An escapement is a device that rotates a wheel in fixed intervals while continually applying intermittent force to maintain the oscillation of the regulator.
Early mechanical clocks used a crown wheel escapement. Later improvements in escapement mechanisms led to greater accuracy and durability. While it is swinging freely, the regulator is isochronous, but if it is left alone, its amplitude will gradually decrease until eventually it comes to a stop.
To keep a clock moving, some force must be regularly applied to the regulator. This action is performed by the escapement. However, the action of the escapement applying force to the regulator disrupts the isochronism of the regulator and reduces its accuracy. Thus, the key to achieving increased accuracy is to make the time in which the force is applied as short as possible. Generally, a foliot balance comprises a rod with toothlike projections along its top and weights hanging from both ends, which swings back and forth horizontally.
At the center of the foliot balance is a rotating shaft with two protruding pallets attached at the top and bottom. The crown wheel escapement comprises a crownlike wheel escape wheel and the two pallets connected to the rotating shaft of the foliot balance.
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