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 timepiece 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 a weight, a mainspring, or an electric current. In mechanical clocks, periodic lifting of the weight or tightening of the spring is 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 transmits the spring’s energy to the hands by means of a gear train, with a balance wheel regulating the motive force.

III

Electric and Electronic Timepieces

In the electric clocks used in homes today, a small motor runs in synchrony with the power-station generator, which is regulated to deliver an alternating current of precise frequency. 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 that is connected to an electrical circuit that induces oscillation at about 100,000 Hz (hertz, or cycles per second). The high-frequency oscillation sets up an alternating current, which is reduced to a frequency more convenient for time measurement and recording, and is thus made to drive the motor that controls the movement of the hands of the clock or watch. The maximum error of the most accurate quartz-crystal clocks is plus or minus one second in ten years.

Electric and electronic watches are powered by small batteries that function for about a year without replacement. The battery may be used to stimulate the balance wheel oscillations of an otherwise mechanical clock, or it may be used to drive the oscillations of a small tuning fork or, most commonly, 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 jewellers for calibrating measuring devices. The first successful chronometer was constructed in 1761 by the English horologist John Harrison. This was a portable instrument that had a compensation mechanism to ensure that the length of the pendulum was independent of temperature; it was also mounted on gimbals to maintain the delicate mechanism in a level position.

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 chronograph exist, including the tachometer, which measures speed of rotation; the pulsometer, which determines a pulse rate; and the stopwatch, a form of chronograph used in athletic contests, which shows elapsed time without providing the time of day.