Clocks and Watches

V

Atomic Clocks

The most precise timekeeping devices are atomic clocks. Their uses include measuring the rotation of the earth, which may vary by 4 to 5 milliseconds per day, and aiding navigational systems such as the global positioning system in computing distances. Atomic clocks are tuned to the frequency of the electromagnetic waves that are emitted or absorbed when certain atoms or molecules make the transition between two closely spaced, or hyperfine, energy states. Because the frequency of these waves is unaffected by external forces, the corresponding period of the waves can be used as a standard to define time intervals.

The cesium-atom clock is used to define the second, the basic unit of time of the International System of Units. In this clock, cesium-133 atoms in one hyperfine energy state are subjected to microwave radiation that is near the resonant frequency of the transition to another hyperfine energy state. The microwave frequency is adjusted, and when the correct frequency is reached, many atoms make the transition to the new energy state. The frequency of the microwave radiation is then used to determine the period of the microwave, or the time interval between wave crests. The second is defined as the duration of 9,192,631,770 periods of radiation. 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.

The hydrogen clock and the ammonia clock rely on the maser principle. In a hydrogen clock, a focused magnetic field selects hydrogen atoms in a specific hyperfine energy state. These atoms are forced to change to a lower energy state. When many atoms make the transition, they begin to oscillate between the two states, emitting energy in the form of an electromagnetic wave. The period of this emitted wave is used to measure time. The hydrogen clock is very stable for several hours at a time.

---

---

VI

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 3rd century bc by Chaldean astronomer Berossus. 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.

A

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.

B

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, 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.

C

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, 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.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain