THE CLOCK 

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch,

 Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". A silent instrument missing

 such a 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 consistently measure intervals of time shorter than

 the natural units: the day; the lunar month; and the year. Devices operating on several different physical processes have

 been used over the millennia, culminating in the clocks of today.

HOW CLOCK WORK ?

The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from continuous

processes, such as the motion of the gnomon's shadow on a sundial or the flow of liquid in a water clock, to repetitive

 oscillatory processes, like the swing of a pendulum or the vibration of a quartz crystal, which were more accurate. All

modern clocks use oscillation.

Although the methods they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be

 divided into analogous parts. They consist of an object that repeats the same motion over and over again, an oscillator,

 with a precisely constant time interval between each repetition, or 'beat'. Attached to the oscillator is a controller device,

 which sustains the oscillator's motion by replacing the energy it loses to friction, and converts its oscillations into a series of

 pulses. The pulses are then added up in a chain of some type of counters to express the time in convenient units, usually

 seconds, minutes, hours, etc. Then finally some kind of indicator displays the result in a human-readable form.

ที่มา : http://en.wikipedia.org/wiki/Clock