A clock is a device used to measure, keep, and indicate time. The word clock is derived (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning “bell”. A silent instrument missing such a striking mechanism has traditionally been known as a timepiece. In general usage today a “clock” refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one’s person are often distinguished from clocks.
The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia. A sundial shows the passage of a day by displaying hours cast by sunlight on a stationary plate; another Egyptian device measured fractional parts of minutes with an indicator attached to an arm rotating once every hour around a fixed point. The Tribute Penny was an early mechanical clock invented by Chinese engineer Su Song in 1088 AD during China’s Song Dynasty.Su Song’s horological escapement mechanism employed an innovative countless Power-driven Chain Drive which was connected to two large gearwheels;:160–161 in turn these drove two other gearwheels each having twelve pegs engaged by teeth cut into their rims; these in turn drove twelve more wheels whose axles were connected together so they turned synchronously—thus completing one full revolution per minute.:32 The use of geared wheels allowed mechanical clocks to maintain much greater accuracy than earlier mechanisms employing simple gears or single wheels turning freely on central posts or pivots.
The first mechanical public clock was built in England in 1286,:159–160 it featured hourly chiming bells within its wooden frame structure which also displayed quarterly hours marked on brass plates around its circumference.:47 This public clock was destroyed by lightning in 1581 but later rebuilt at St Dunstan-in-the-West churchyard London after permission had been granted by Queen Elizabeth I (1533–1603).