clock

Photograph of a clock made by English clockmaker Thomas Tompion, ‘the father of English clockmakers’, in about 1676. Tompion invented the escapement with a horizontal wheel that made it possible to construct a flat watch. His clock can be seen at Greenwich Observatory, London, England.

Any device that measures the passage of time, usually shown by means of pointers moving over a dial or by a digital display. A small timepiece worn on the wrist or carried is a watch. Nearly all modern clocks and watches are electronic and controlled by a vibrating quartz crystal. Mechanical timepieces are still made, however. Mechanical clocks consist of a train of wheels driven by a weight or by electricity and controlled by a pendulum. Mechanical watches are driven by a spring and controlled by a balance spring.

History In ancient Egypt the time during the day was measured by a shadow clock, a primitive form of sundial, and at night the water clock was used. Up to the late 16th century the only clock available for use at sea was the sand clock, of which the most familiar form is the hourglass. During the Middle Ages various types of sundial were widely used, and portable sundials were in use from the 16th to the 18th century. Watches were invented in the 16th century - the first were made in Nuremberg, Germany, shortly after 1500 - but it was not until the 19th century that they became cheap enough to be widely available. The first known public clock was set up in Milan, Italy, in 1353. The timekeeping of both clocks and watches was revolutionized in the 17th century by the application of pendulums to clocks and of balance springs to watches.

Types of clock The marine chronometer was a mechanical precision timepiece of special design, used at sea for giving Greenwich mean time (GMT). Electric timepieces were made possible by the discovery early in the 19th century of the magnetic effects of electric currents. One of the earliest and most satisfactory methods of electrical control of a clock was invented by Matthaeus Hipp in 1842. In one kind of electric clock, the place of the pendulum or spring-controlled balance wheel is taken by a small synchronous electric motor, which counts up the alternations (frequency) of the incoming electric supply and, by a suitable train of wheels, records the time by means of hands on a dial. The quartz crystal timepiece (made possible by the piezoelectric effect of certain crystals) has great precision, with a short-term variation in accuracy of about one-thousandth of a second per day. The atomic clock, used in scientific work, utilizes the natural resonance of certain atoms (for example, caesium) as a regulator controlling the frequency of a quartz crystal oscillator. Atomic clocks can be accurate to within one second in 30 million years (1 part in 1015).

In 2005 an atomic clock was developed based on the frequency of strontium atom oscillations. Lasers were used to separate individual strontium atoms electromagnetically into a series of energy wells, preventing their oscillations form interfering with each other. This technique allowed the oscillations of a number of atoms to be measured at once, increasing the accuracy of the clock. The strontium ‘optical lattice clock’ can potentially keep time with an accuracy of 1 part in 1018, or one second in 30 billion years.