Clocks are mechanical devices used to measure or indicate the passage of time.
The clock requires a source of power and a mechanical means of transmitting and controlling it, as well as indicators to register lapse of time units. 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 force generated by the power source is transmitted by a gear train and regulated by a pendulum or a balance wheel. 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.