Clocks and Watches

I

Introduction

Clocks and Watches, 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.

II

Mechanisms

In a clock, the source of 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.

III

Electric Timepieces

In the electric clocks used in homes today, a small motor runs in unison with the power-station generator, which is 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.

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The quartz-crystal clock developed in 1929 for precision timekeeping employs a ring of quartz that is connected to an electrical circuit and made to oscillate between 10,000 and 100,000 hertz (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 or a digital display. 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 one 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.

IV

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 English horologist John Harrison. 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 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.

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