Clock

Clock A device for indicating the passage of time. The mechanical clock has a power source, such as a falling weight or spring, whose energy is released slowly and at a measured rate by a regulator, such as a balance wheel or pendulum. The gradual mevement of the spring or fall of the weight is conveyed by a train of gears to hands that indicate the time. The first mechanical clocks appeared in Europe in the 13th century.
But clocks did not become reasonably accurate until the mid-1600s when Christian Huygens first used a pendulum as a regulator. In the 1670s the introduction of a balance wheel for regulation led to the development of compact portable clocks, or watches.

Atomic Clock The most accurate of all clock, which is regulated by the precise vibration of certain atoms or molecules. An atomic clock my gain or lose less than a second in 100,000 years.

Chronometer An accurate timepiece. The original chronometers were developed to aid navigation at sea. The first accurate instrument was invented by Yorkshireman John Harrison in 1759.

Clepsydra or water clock A device dating back at least to ancient Egyptian times which used the flow of water from a vessel to measure the passage of time.

Sundial An early device for telling the time by means of shadows. It consists of a pointer, or gnomon, which casts the shadow, set in the middle of a flat dial. The gnomon must slant upwards so that it makes an angle with the dial face equal to latitude of the location.

Quartz watch A Watch regulated by the vibrations of a wafer-thin quartz crystal. When a tiny current is applied to the crystal, it vibrates exactly 32,768 times a second. A microcircuit converts this 'time base' in the quartz digital watch to a digital dispaly, such as LCD or LED.
In the Quartz analog watch the time base drives a gear train to move the hands.