Clock

A clock (from the Latin cloca, "bell") is an instrument for measuring time. A clock is usually a physical instrument; an especially accurate one is called a chronometer. The clock in its modern form (24 hour clock) has been in use since at least the 14th century. Clock can refer to an abstract system of time measurement (see calendar for other measurements). Modern clocks define constant units of time: an hour is always sixty minutes, of sixty seconds each.

Purposes

Clocks are in homes and offices; smaller ones (watches) are carried; big ones are in public places, e.g. a train station or church. A small clock is often shown in a corner of computer displays or mobile phones.

Related Topics:
Watch - Train station - Church - Computer display - Mobile phone

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The main purpose of a clock is not always to display the time.

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It may also be used to control a device according to time, e.g. an alarm clock, a VCR, or a time bomb (see: counter).

Related Topics:
VCR - Bomb - Counter

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Practically all computers depend on an accurate internal clock signal to allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuits). Some computers also maintain time and date for all manner of operations whether these be for alarms, event initiation or just to display the time of day.

Related Topics:
Computer - Clock signal - Asynchronous circuit

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Ideal clocks

An ideal clock is a scientific principle that measures the ratio of the duration of natural processes, and thus will give the time measure for use in physical theories. Therefore, to define an ideal clock in terms of any physical theory would be circular. An ideal clock is more appropriately defined in relationship to the set of all physical processes.

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This leads to the following definitions:

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• A clock is a recurrent, periodic process and a counter.
• A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
• An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.

The recurrent, periodic process (a metronome) is an oscillator and typically generates a clock signal. Sometimes that signal alone is (confusingly) called "the clock," but sometimes "the clock" includes the counter, its indicator, and everything else supporting it.

Related Topics:
Metronome - Oscillator

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This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances. While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within current measurement tolerances, they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical processes. However, they are not so designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition.

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