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Ancient methods of measuring hours in the absence of sunlight included the notched candle and the Chinese practice of burning a knotted rope and noting the length of time required for the fire to travel from one knot to the next. Devices almost as old as the shadow clock and sundial include the hourglass, in which the flow of sand is used to measure time intervals, and the water clock, or clepsydra, in which the flow of water indicates passage of time. Clepsydras became more complicated, even to the inclusion of gearing in about 270 bc by Greek inventor Ctesibius of Alexandria. Eventually, a weight falling under the force of gravity was substituted for the flow of water in time devices, anticipating the mechanical clock. The historical origin of the mechanical clock is obscure. The first recorded examples are found in the 14th century. Until that time, a time-measuring instrument was known as a horologium, or hour teller. The name clock, which originally meant “bell,” was first applied in the present sense to the huge, mechanical time indicators installed in bell towers in the late Middle Ages. Clockworks were initially heavy, cumbersome devices. A clock built in the 14th century by Henry De Vick of Württemberg for the royal palace (now the Palais de Justice) in Paris was powered by a 227-kg (500-lb) weight that descended a distance of 9.8 m (32 ft). The apparatus for controlling its rate of fall was crude and the clock inaccurate. Clocks of that period had dials with only one hand, which indicated the nearest quarter hour. A series of inventions in the 17th and 18th centuries increased the accuracy of clockworks and reduced the weight and bulk of the mechanisms. Galileo had described late in the 16th century the property of a pendulum, known as isochronism, stating that the period of the swing is constant. In 1657 Dutch physicist Christiaan Huygens showed how a pendulum could be used to regulate a clock. Ten years later English physicist Robert Hooke invented an escapement, which permitted the use in clocks of a pendulum with a small arc of oscillation. British clockmaker George Graham improved the escapement, and John Harrison developed a means of compensating for variations in the length of a pendulum resulting from changes in temperature. Watchworks were developed when coiled springs were introduced as a source of power. This type of spring was used in Italy about 1450. About 1500 Peter Henlein, a locksmith in Nürnberg, Germany, began producing portable timepieces known popularly as Nürnberg eggs. In 1525 another artisan, Jacob Zech of Prague, invented a fusee, or spiral pulley, to equalize the uneven pull of the spring. Other improvements that increased the accuracy of watches included a spiral hairspring, invented about 1660 by Robert Hooke, for the balance wheel, and a lever escapement devised by British inventor Thomas Mudge about 1765. Minute and second hands, and crystals to protect both the dial and hands, first appeared on 17th-century watches. Jeweled bearings to reduce friction and prolong the life of watchworks were introduced in the 18th century. In the centuries that preceded the introduction of machine-made parts, craftsmanship of a high order was required to manufacture accurate, durable clocks and watches. Such local craft organizations as the Paris Guild of Clockmakers (1544) were organized to control the art of clockmaking and its apprenticeship. A guild known as the Clockmakers Company, founded in London in 1630, is still in existence. The Netherlands, Germany, and Switzerland also produced many fine artisans whose work was noted for beauty and a high degree of mechanical perfection. The clock was often a decorative as well as a useful instrument. Early clocks were highly ornamented. Many bore sculptured figures, and clockworks were used in the towers of late medieval Europe to set in motion huge statues of saints or allegorical figures. Cuckoo clocks, containing carved wooden birds, which emerge and “sing” to tell the time, were made in the Black Forest of Germany as early as 1730 and are still popular. Some early English clocks were made in the form of lanterns or birdcages. The grandfather, or case, clock, which has the pendulum and weight exposed beneath a gear housing at the top of a tall cabinet, was designed before machine-cut gears were introduced, and it continues to be a popular ornamental clock. Watches were originally shaped like drums or balls and were worn suspended from a belt or kept in a pocket . Wristwatches became popular as watchworks became smaller. Beginning in the 18th century, Switzerland became the center of a watchmaking industry, particularly in the villages of the Jura Mountains. At first a cottage industry, with families manufacturing watch parts at home to be assembled and sold by a master watchmaker, Swiss watchmaking by the 1850s had led to the development of a number of small factories and the foundation of a major industry. Some modern Swiss watchworks are tiny enough to fit into pencil ends or in earrings. European clockmakers and watchmakers brought their skills and mechanical ingenuity to colonial America. During this period, however, Chauncey Jerome of Bristol, Connecticut, devised a rolled-brass clock movement that could be sold at a low price. Such innovations, together with the economies of mass production, soon made the United States the leading clock-making country of the world. As production increased, competition reduced the price of a clock to $1 or less, and for the first time most families could afford a clock. Watches also became cheaper as production rose. American horologists Aaron Dennison and Edward Howard, working in Massachusetts, invented and perfected automatic production machinery in the 1850s. New designs reduced the number of parts required. Watches wound with keys were replaced after 1875 by stem-wound types. The first Waterbury, a famous American pocket watch, could be sold for only $4 because it used a stamped-out mechanism without jewels. Later watches were even less expensive. The Ingersol and the Ingraham, for example, became known as the dollar watches. The electric clock was an American innovation of the early 1900s, invented by Henry E. Warren, who induced producers of electric power to time the alternating-current cycles carefully so that synchronous motors could be used for clocks. The invention by W. H. Shortt in 1921 of the Shortt Free Pendulum, first installed in the Edinburgh Observatory, made possible the most accurate timekeeper until the introduction of the quartz clock in the United States in 1929. The first improvement over the quartz clock was the cesium atomic clock, developed in England in 1955. Electric wristwatches appeared on the market in 1957, followed in 1959 by an electronic watch that substituted a small tuning fork for the usual escapement, with a battery to power the transistorized oscillating circuit. More recent developments have been the LED (light-emitting diode) and LCD (liquid crystal display) watches. The LED, developed in the 1960s, uses the light-producing characteristics of certain semiconductors to illuminate its digital time display; a quartz crystal provides the oscillations that are reduced to compute time. The LCD, produced in the 1970s, uses liquid crystals, materials having optical properties similar to liquids and solid crystals. Scientific advances in metallurgy and other fields have led to many improvements in timekeeping devices of all types. The mainsprings of present-day mechanical watches are made from metals that resist breakage and rust, synthetics have replaced precious stones in jeweled bearings, and cases have been perfected that seal out both dust and moisture. Other special-purpose watches include the Braille watch for the blind, which has sturdy hands not covered with a crystal, and raised dots on the dial to mark the hours; the alarm watch for the pocket or wrist, which functions as a tiny, portable alarm clock; and the calendar watch, which shows the day of month and the week. New sources of power, such as sunlight, body heat, and atomic energy, are being investigated in current horological research. The cesium-atom clock is very accurate and remains stable over long periods of time. The most stable cesium-atom clocks have an error of about plus or minus one second in one million years. The rubidium clock uses the transition of the rubidium-87 atom between two hyperfine energy states. It employs the same basic principle as the cesium-atom clock. The rubidium atoms, however, are first forced to change their hyperfine energy state and are then subjected to microwave radiation to return them to their original state. When many atoms return to their original state, the correct transition frequency has been reached and the period of the wave can be used to measure time. Rubidium clocks are not as stable or as accurate as cesium-atom clocks, but they are more compact and less expensive.
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