The Evolution of Mechanical Clocks

The operation of a clock depends on a stable mechanical oscillator, such as a swinging pendulum or a mass connected to a spring, by means of which the energy stored in a raised weight or coiled spring advances a pointer or other indicating device at a controlled rate. It is not definitely known when the first mechanical clocks were invented. Some authorities attribute the first weight-driven clock to Pacificus, archdeacon of Verona in the 9th cent. Gerbert, a learned monk who became Pope Sylvester II, is often credited with the invention of a mechanical clock, c.996.

Mechanical figures that struck a bell on the hour were installed in St. Paul's Cathedral, London, in 1286; a dial was added to the clock in the 14th cent. Clocks were placed in a clock tower at Westminster Hall, London, in 1288 and in the cathedral at Canterbury in 1292. In France, Rouen was especially noted for the skill of its clockmakers and watchmakers. Probably the early clock closest to the modern ones was that constructed in the 14th cent. for the tower of the palace (later the Palais de Justice) of Charles V of France by the clockmaker Henry de Vick (Vic, Wieck, Wyck) of Württemburg. Until the 17th cent. few mechanical clocks were found outside cathedral towers, monasteries, abbeys, and public squares.

The early clocks driven by hanging weights were bulky and heavy. When the coiled spring came into use (c.1500), it made possible the construction of the smaller and lighter-weight types. By applying Galileo's law of the pendulum, the Dutch scientist Christiaan Huygens invented (1656 or 1657) a pendulum clock, probably the first. Early clocks used in dwellings in the 17th cent. were variously known as lantern clocks, birdcage clocks, and sheep's-head clocks; they were of brass, sometimes ornate, with a gong bell at the top supported by a frame. Before the pendulum was introduced, they were spring-driven or weight-driven; those driven by weights had to be placed on a wall bracket to allow space for the falling weights. These clocks, probably obtained chiefly from England and Holland, were used in the Virginia and New England colonies.

Clocks with long cases to conceal the long pendulums and weights came into use after the mid-17th cent.; these were the forerunners of the grandfather clocks. With the development of the craft of cabinetmaking, more attention was concentrated on the clock case. In France the tall cabinet clocks, or grandfather clocks, were often of oak elaborately ornamented with brass and gilt. Those made in England were at first of oak and later of walnut and mahogany; simpler in style, their chief decoration was inlay work.

Electric and Other Clocks

Electric clocks were made in the second half of the 19th cent. but were not used extensively in homes until after c.1930. In an analog clock the hands of an electric clock are driven by a synchronous electric motor supplied with alternating current of a stable frequency. Digital clocks use LCDs (liquid crystal displays) or LEDs (light emitting diodes) to form the numbers indicating the time. The quartz clock, invented c.1929, uses the vibrations of a quartz crystal to drive a synchronous motor at a very precise rate. Some quartz clocks have an error of less than one thousandth of a second per day. The atomic clock, which is based upon the frequency of an atomic or molecular process, is even more precise; a state of the art atomic clock, such as the NIST-F1 (which is the U.S. time frequency standard clock), neither gains nor loses a second in 20 million years.

Some Famous Clocks

One of the most famous clocks is in the cathedral of Strasbourg; the clock was first placed in the cathedral in 1352, and in the 16th cent. it was reconstructed. In the 19th cent. a new astronomical clock (so called because it shows the current positions of the sun, moon, and other heavenly bodies in addition to the time of day) similar to the original clock was constructed; its elaborate mechanical devices include the Twelve Apostles, a crowing cock, a revolving celestial globe, and an automatic calendar dial. Among other well-known clocks of the world are the clock known as Big Ben in the tower next to Westminster Bridge in the British Houses of Parliament and the tower clock in the Metropolitan Life Insurance Company building, New York City.

The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error between a pair of such clocks, i.e., the difference in indicated time if both were started at the same instant and later compared, was typically about one second in three thousand years. In 1955 the first cesium-beam clock (a device that uses as a reference the exact frequency of the microwave spectral line emitted by cesium atoms) was placed in operation at the National Physical Laboratory at Teddington, England. It is estimated that such a clock would gain or lose less than a second in three million years. The U.S. standard is the NIST-F1, which went into service in 1999 and should neither gain nor lose a second in 20 million years. A fountain atomic clock, the NIST F-1 consists of a 3-foot vertical tube inside a taller structure. It uses lasers to cool cesium atoms, forming a ball of atoms that lasers then toss into the air, much like one would toss a tennis ball, creating a fountain effect. This allows the atoms to be observed for much longer than could be done with any previous clock.

Many of the world's nations maintain atomic clocks at standards laboratories, the time kept by these clocks being averaged to produce a standard called international atomic time (TAI). Highly accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites, the signals being used for such things as tracking space vehicles, electronic navigation systems, and studying the motions of the earth's crust. The accuracy of these clocks made possible an experiment confirming an important prediction of Einstein's theory of relativity. Prototypes of atomic clocks using atoms such as hydrogen or beryllium could be still thousands of times more accurate. For example, researchers at the U.S. National Institute of Standards and Technology have demonstrated an atomic clock based on an energy transition in a single trapped mercury ion (a mercury atom that is missing one electron) that has the potential to be up to 1,000 times more accurate than current atomic clocks.

Ancient device for measuring time by the gradual flow of water. One form, used by North American Indians and some African peoples, consisted of a small boat or floating vessel that shipped water through a hole until it sank. In another form, water escaped through a hole in a vessel marked with graduated lines; specimens from Egypt date from the 14th century BC. The Romans invented a clepsydra consisting of a cylinder into which water dripped from a reservoir; a float provided readings against a scale on the cylinder wall. Galileo used a mercury clepsydra to time his experimental falling bodies. Seealso clock.

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