A clock is an instrument by means of which we may divide a day into small parts of time and make these parts visible.

Iohann Litrov. The sky mysteries. 1834.

A clock is a tool for the time counting. The stable repeating processes are used in clocks. They are: the Earth rotation (a solar clock), a pendulum oscillating (mechanical and electronic clocks), a tuning fork (turning fork clocks), atoms transitions from one energetic condition to another (quantum clock). The clock time measuring is the periods number measuring of used process (for example, a pendulum oscillating). All clocks are conditionally divided into timepieces (pocket, wristwatch,desk,clocks,ets) and special timepieces (stopwatch chronometers). The first timepieces were sand glasses; in the 2nd thousand years B.C. there occured water clocks; the first mentions about mechanical timepieces related to the end of the 6th century; the modern mechanical timepieces are created by H. Huigens in 1657; in 70th years of the 20th century there were widespread the electronic quartz watches with numeric indication, for example on the liquid chips.

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