Clock
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For other uses, see Clock (disambiguation).
Platform clock at King's Cross railway station, London.
Platform clock at King's Cross railway station, London.
A clock is an instrument for measuring, indicating and maintaining the time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning REMINDER SYSTEM "bell". For horologists and other specialists the term clock continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing a bell, a set of chimes, or a gong.[citation needed] A silent instrument lacking such a mechanism has traditionally been known as a timepiece.[1] In general usage today, however, a "clock" REMINDER SYSTEM refers to any device for measuring and displaying the time which, unlike a watch, is not worn on the person.
Clock at the Royal Observatory, Greenwich
Clock at the Royal Observatory, Greenwich
Contents
[hide]
* 1 History
o 1.1 Sundials and other devices
REMINDER SYSTEM o 1.2 Water clocks
o 1.3 Early clocks
+ 1.3.1 A new mechanism
+ 1.3.2 Early astronomical REMINDER SYSTEM clocks
o 1.4 Later developments
* 2 How clocks work
o 2.1 Power source
o 2.2 Oscillator
o 2.3 REMINDER SYSTEM Controller
o 2.4 Counter chain
o 2.5 Indicator
* 3 Types
o 3.1 Time display methods
REMINDER SYSTEM + 3.1.1 Analogue clocks
+ 3.1.2 Digital clocks
+ 3.1.3 Auditory clocks
* 4 Purposes
REMINDER SYSTEM o 4.1 Ideal clocks
o 4.2 Navigation
* 5 Seismology
* 6 Specific types of clocks
* 7 See also
* 8 Notes
* 9 References
* 10 External REMINDER SYSTEM links
[edit] History
Further information: History of timekeeping devices
Replica of an ancient Chinese incense clock
Replica of an ancient Chinese incense clock
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. Such measurement requires devices. Devices REMINDER SYSTEM operating on several different physical processes have been used over the millennia, culminating in the clocks of today.
[edit] Sundials and other devices
The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued REMINDER SYSTEM to be used to monitor the performance of clocks until the modern era. However, its practical limitations - it requires the sun to shine and does not work at all during the night - encouraged the use of other techniques for measuring time.
Candle clocks and sticks of incense that burn down at, approximately, predictable speeds have also been REMINDER SYSTEM used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.
[edit] Water clocks
Main article: Water clock
A scale model of Su Song's Astronomical Clock Tower, built in 11th century Kaifeng, China. It was driven REMINDER SYSTEM by a large waterwheel, chain drive, and escapement mechanism.
A scale model of Su Song's Astronomical Clock Tower, built in 11th century Kaifeng, China. It was driven by a large waterwheel, chain drive, and escapement mechanism.
Water clocks, also known as clepsydrae(sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical REMINDER SYSTEM gnomon and the day-counting tally stick.[2] Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, REMINDER SYSTEM also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.[3]
The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and also resulted REMINDER SYSTEM in improved accuracy. These advances were passed on through Byzantium and Islamic times, eventually making their way to Europe. Independently, the Chinese developed their own advanced water clocks, passing their ideas on to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. It is important to point out that the need for the common person to 'know what time it is' largely did not exist until the Industrial Revolution, when it became important to keep track of hours worked. In the earliest of times, however, the purpose for using a water clock was for astronomical and astrological reasons. These early water clocks were calibrated with a sundial. Through the centuries, water clocks were used for timing lawyer's speeches during a trial, labors of prostitutes, night watches of guards, sermons and Masses in church, to name only a few. While never reaching the level of accuracy based on today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.
