devices used to measure or indicate the passage of time. A clock, which is larger than a watch, is usually intended to be kept in one place; a watch is designed to be carried or worn. Both types of timepieces require a source of power and a means of transmitting and controlling it, as well as indicators to register the lapse of time units.

Mechanisms.

In a clock the source of its power may be produced by weight, a mainspring, or an electric current. Except in electric or electronic clocks, periodic adjustments, such as lifting the weight or tightening the spring, are needed. The motive force generated by the power source in a mechanical clock is transmitted by a gear train and regulated by a pendulum or a balance wheel. In such a clock the time may be reported audibly by the striking of a gong or chime and is registered visually by the rotation of wheels bearing numerals or by the position of hands on a dial; in electric or electronic clocks, time may be shown by a display of numbers.

A mechanical watch uses a coiled spring as its power source. As in spring-powered clocks, the watch conserves energy by means of a gear train, with a balance wheel regulating the motive force. In self-winding watches the mainspring is tightened automatically by means of a weight on a rotor that responds to the arm movements of the wearer.

Electric Timepieces.

In the electric clocks used in homes today, a small motor runs in unison with the power-station generator, regulated to deliver an alternating current of precisely 60 cycles per second. Electric currents may also be used to keep the movements of several “slave” clocks synchronized with the pendulum in a master clock.

The quartz-crystal clock developed (1929) for precision timekeeping employs a ring of QUARTZ, (q.v.) that is connected to an electrical circuit and made to oscillate at 100,000 Hz (cycles per second). The high frequency oscillation is converted to an alternating current, reduced to a frequency more convenient for time measurement, and thus made to drive the motor of a synchronous clock. The maximum error of the most accurate quartz-crystal clocks is plus or minus one second in ten years.

The electric or electronic watch is powered by a small battery that functions for about a year without replacement. The battery may drive the balance wheel of an otherwise mechanical clock, or it may be used to drive the oscillations of either a small tuning fork or a quartz crystal.

Chronometers.

Carefully constructed mechanical timepieces known as chronometers are precision devices used by navigators in the determination of their longitude at sea and by astronomers and jewelers for calibrating measuring devices. The first successful chronometer was constructed in 1761 by the English horologist John Harrison (1693–1776). These portable instruments are mounted on a box on gimbals so as to maintain the delicate movements in a level position. The modern wrist chronometer is a precision watch regulated in different positions and at various temperatures and certified by testing bureaus in Switzerland.

Another precision timekeeper is the chronograph, which not only provides accurate time but also registers elapsed time in fractions of a second. Various forms of chronographs exist, including the telemeter, which measures the distance of an object from the observer; the tachometer, which measures speed of rotation; the pulsometer, which determines a pulse rate; and the production counter, which indicates the number of products made in a given time. The timer, or stopwatch, a form of chronograph used in athletic contests, shows elapsed time without providing the time of day.

Atomic Clocks.

The most precise timekeeping devices are atomic clocks, which are tuned to the frequency of oscillation between two energy states of certain atoms or molecules. These vibrations are unaffected by external forces. Operation of the cesium-atom clock, used to define the basic unit of time of the INTERNATIONAL SYSTEM OF UNITS (q.v.), depends on measurement of the frequency of the energy absorbed by a cesium atom in undergoing the change from a lower energy level to a higher one.

The ammonia clock and the hydrogen clock rely on the MASER, (q.v.) principle. The ammonia maser separates ammonia molecules into two different energy levels, and the high, constant frequency at which the molecules oscillate between one level and the other is used to measure time with great precision. Atomic clocks are used, among other applications, to measure the rate of rotation of the earth, which may vary by 4 to 5 milliseconds from one day to the next. See The Scientific Standard of Time..

Historical Development.

Throughout history, time has been measured by the movement of the earth relative to the sun and stars. The earliest type of timekeeper, dating from as far back as 3500 bc, was the shadow clock, or gnomon, a vertical stick or obelisk that casts a shadow. An Egyptian shadow clock of the 8th century bc is still in existence. The first hemispherical sundial was described about the 3d century bc by the Chaldean astronomer Berossus (fl. 300–260 bc). Ancient methods of measuring hours in absence of sunlight included 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, and the notched candle. 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 (q.v.), 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 the Greek inventor Ctesibius of Alexandria (fl. 270–246 bc). Eventually a weight falling under the force of gravity was substituted for the flow of water in time devices, anticipating the mechanical clock.

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 (fl. 1360) 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.

The pendulum.

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, (q.v.), known as isochronism, stating that the period of the swing is constant. In 1657 the Dutch physicist Christiaan Huygens showed how a pendulum could be used to regulate a clock. Ten years later the English physicist Robert Hooke invented an escapement, which permitted the use in clocks of a pendulum with a small arc of oscillation. The British clockmaker George Graham (1673–1751) improved the escapement, and John Harrison developed a means of compensating for variations in the length of a pendulum resulting from changes in temperature.

Watches.

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 (1480–1542), a locksmith in Nüremberg, Germany, began producing portable timepieces known popularly as Nüremberg 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 the British inventor Thomas Mudge (1717–94) 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 Co., 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.

Decorative clocks.

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-wood 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; 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.

Production in the U.S.

European clockmakers and watchmakers brought their skills to colonial America and originated much of the mechanical ingenuity that has distinguished American industry. In 1650, before the introduction of the pendulum clock, a clock could be found in a Boston church tower. The first public clock in New York City was built in 1716 for the City Hall at Nassau and Wall streets, and a clock was installed in Independence Hall in Philadelphia by 1753.

Mass production of clocks with interchangeable parts began in the U.S. after the American Revolution. Because of the scarcity of metals, well-seasoned wood was utilized for the movements. In the early 1800s, Simon Willard (1753–1848) of Roxbury, Mass., patented the popular banjo clock, and Eli Terry (1772–1852) of Connecticut evolved a shelf clock called the pillar-and-scroll clock, which required winding only once a day. About the same time in Plymouth Hollow (now Thomaston), Conn., Seth Thomas (1785–1859) founded the Seth Thomas Clock Co., which was, in the mid-20th century, one of the largest clock factories in the world.

Watches were not produced in significant volume in the U.S. until about 1800, when Thomas Harland (1735–1807) of Norwich, Conn., established a factory with a capacity of 200 units a year. In 1836 the Pitkin brothers of East Hartford, Conn., produced the first American-designed watch and the first containing a machine-made part. Despite a reputation for accuracy and durability, the manufacture of this watch was discontinued as a result of the depression of 1837, which temporarily crippled American industry.

During this period, however, Chauncey Jerome (1793–1868) of Bristol, Conn., 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 U.S. the leading clockmaking country of the world. As production increased, competiton 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. The American horologists Aaron Dennison (1812–95) and Edward Howard (1813–1904), 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.

Recent Developments.

The electric clock was an American innovation of the early 1900s, invented by Henry E. Warren (1872–1957), 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 Edinburgh Observatory, made possible the most accurate timekeeper up to the introduction of the quartz clock in America 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 (see ELECTRONICS,; LIQUID CRYSTAL,). 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 that functions as a portable, tiny 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.

For further information on this topic, see the Bibliography, section 636. Clocks and watches.