Clock
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- For other uses, see Clock (disambiguation).
A clock (from the Latin cloca, "bell") is an instrument for measuring time. (Usually, for measuring time of intervals less than a day--as opposed to a calendar.) Those used for technical purposes, of very high accuracy, are sometimes called chronometers. A portable clock is called a watch. The clock in its most common modern form (in use since at least the 14th century) displays the hours, minutes, and sometimes seconds that pass over a twelve or twenty-four-hour period.
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History
The clock is one of the oldest human inventions. In principle, it requires no more than some physical process which will proceed at a known rate, and a way to gauge how long that process has been continuing. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes could be used to measure off hours and minutes. The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely known in ancient times. Candles and sticks of incense which burn down at approximately predictable speeds have also been used as clocks. In an hourglass fine sand pours through a tiny hole at a predictable rate.
The historian Vitruvius reported that the ancient Egyptians also used a clepsydra, a time mechanism run by flowing water. Herodotus had mentioned an ancient Egyptian time-keeping device that was based on mercury. Historians disagree over the Antikythera mechanism but this is largely thought to be an early mechanical clock. By the 9th century AD a mechanical timekeeper had been developed that lacked only an escapement mechanism. There is a record that in 1176 Sens Cathedral installed a ‘horologe’—the word still used in French for large clocks. (from Greek hora, hour, and legein, to tell). This word has led scholars to believe that these tower clocks did not employ hands or dials, but “told” the time with audible signals such as bells.
The earliest reasonably accurate clocks are the 13th century tower clocks probably developed for (and perhaps by) monks in Northern Italy. These were used to announce the canonical hours or intervals between set times of prayer. Canonical hours differ in length, and varied as the times of sunrise and sunset shifted.
The world's first self-striking clock was said to be invented by Chang Yeong-Sil, a chief enginner of Korea, in Korea during the Joseon Dynasty. It was called Chagyongru, which means "self-striking clock" in Korean. [1] [2]
The earliest table clocks that survive in any quantity are mid-16th century ones from the metalworking towns of Nuremberg and Augsburg. These clocks have only one time-keeping hand. The dial between the hour markers is divided into four equal parts making the clocks readable to the nearest 15 minutes.
Image:Big.ben.scaled.arp.750pix.jpg
The next major development in accuracy occurred in 1657 with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to propel the motion of a time telling device earlier in the 17th century. Christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (99.38 cm or 39.13 inches for the one second movement) and had the first pendulum driven clock made. In 1670, the English clockmaker William Clement created the anchor escapement, an improvement over Huygens' crown escapement. Within just one generation, minute hands and then second hands were added.
The excitement over the pendulum clock attracted the attention of designers resulting in a proliferation of clock forms. Notably, the longcase clock (also known as the grandfather clock) was created to house the pendulum and works. The English clockmaker William Clement, inventor of the anchor escapement, is credited with developing this form in 1670 or 1671. It was also at this time that clock cases began to be made of wood and clock faces to utilize enamel as well as hand-painted ceramics. On November 17, 1797, Eli Terry received his first patent for a clock. Terry is known as the founder of the American clock-making industry.
The development of electronics in the twentieth century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the behaviour of quartz crystals, the decay of radioactive elements or resonance of polycarbonates. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.
Types
Clocks can be classified by their method of time display, as well as by their method of timekeeping.
Time display methods
Analog clocks
Analog clocks indicate time as an angle. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which also serves as a scale of 60 minutes, and often also as a scale of 60 seconds – though many other styles and designs have been attempted throughout the years, including the 8, 10 and 24 hour clock. The ultimate analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow of its gnomon.
Image:Digital-clock-radio-basic.jpg
Digital clocks
Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:
- the 24-hour notation with hours ranging 00–23;
- the 12-hour notation with AM/PM indicator, with hours indicated as 12AM, followed by 1AM–11AM, followed by 12PM, followed by 1PM–11PM (a notation mostly used in the United States).
Most digital clocks use an LCD or LED display; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change, or power failure, digital clocks without a backup battery or capacitor either start counting from 00:00, or stay at 00:00, often with blinking digits indicating that time needs to be set. Some newer clocks will actually reset themselves based on internet-time servers which in turn are tuned to national atomic clocks.
Auditory clocks
For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. the form is either spoken natural language, (e.g. "The time is twelve thirty-five"), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour like the clock Big Ben).
Textual clocks
Textual clocks present the time visually in the form of natural language. For instance, in English, the time 12:35 could be represented as "Twelve thirty-five". Some versions of these clocks use a more approximate version intended to be relaxing, such as "About twelve thirty" <ref>Project Muse</ref>
Timekeeping methods
Most types of clocks are built around some form of oscillator, an arrangement that goes through an endless sequence of periodic state changes, designed to provide a continuous and stable reference frequency. The periods of this oscillator are then counted and converted into the desired clock display.
- Mechanical clocks use a pendulum of some form as their oscillator, which controls the rotation of a system of gears that drive the clock display.
- Crystal clocks use an electronic quartz crystal oscillator and a frequency divider or counter. Most battery-powered crystal clocks use a 215 Hz = 32.768 kHz oscillator.
- Atomic clocks use a microwave oscillator (maser) tuned by the energy transitions of elements such as caesium, rubidium or hydrogen. These are the most precise clocks available. Atomic clocks based on caesium are used as the official definition of time today.
- Mains power clocks count the 50 or 60 hertz periods of their AC power supply, which is steered for long-term average frequency stability by power generating companies for this purpose.
- Radio clocks periodically activate a built-in radio receiver to automatically adjust their time to a broadcast time signal. The broadcast radio signals received are typically generated by an atomic clock located at the transmitter site. These clocks are used extensively by mariners, especially short-wave radio clocks which use simultaneous bursts of time-signals, often encoded or encrypted – not to be confused with number stations.
- Sundials observe the apparent rotation of the Sun around the Earth as their reference oscillation.
Purposes
Clocks are in homes and offices; smaller ones (watches) are carried; larger 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.
The main purpose of a clock is not always to display the time. 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). Though, in this context, it is more appropriate to refer to it as a timer or trigger mechanism rather than strictly as a clock.
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. The accuracy of the internal computer clock is generally maintained by a small battery which powers the internal clock. Memory of this kind is often referred to as "non-volatile". Many computers will still function, even if the internal clock battery is dead, but the computer clock will need to be reset each time the computer is restarted, since once power is lost, time is also lost.
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. Image:PDphotos-org alarm clock 1 bg 050304.jpg
This leads to the following definitions:
- 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.
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.
Navigation
Accurate navigation by ships beyond the sight of land depends on the ability to measure latitude and longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrison created the first, highly accurate marine chronometers in the mid-18th century. The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers.
Modern clocks
Quartz timepieces were invented in the 1920s.
The digital clock was invented in 1956.
Specific types of clocks
- Alarm clock
- Analog clock
- Analog clock with digital display
- Astronomical clock
- Atomic clock
- Binary clock
- Bracket clock
- Cartel clock
- Chiming clock
- Clock network
- Clock of the Long Now
- Countdown clock
- Cuckoo clock
- Data clock for timescapes created with time-technology
- Doll's head clock
- Flip clock
- Game clock
- Hourglass
- Longcase (or "grandfather") clock
- Mantel clock
- Pedestal clock
- Pendulum clock
- Projection clock
- Quartz clock
- Railroad chronometers
- Sidereal clock
- Skeleton clock
- Stopwatch
- Striking clock
- Sundial
- Tide clock
- Time clock
- Torsion pendulum clock
- Watch
- Water clock
- World clock
See also
- Allan variance
- Biological clocks
- Chronometer
- Clock face
- Clockmaker
- Clock of the Long Now
- Clock signal (digital circuits)
- Clock tower
- Colgate Clock, the world's largest clock
- Cox's timepiece
- Horology
- Intellectual history of time
- Iron Ring Clock
- Jens Olsen's World Clock
- Metrology
- National Association of Watch and Clock Collectors
- Radio clock
- Star clock
- Timeline of time measurement technology
- Timer
- Time standard
- Time to digital converter
- Watchmaker
External links
- National Association of Watch and Clock Collectors
- American Watchmakers-Clockmakers Institute
- British Horological Institute
- Illustrated Glossary of Clock Types
- London's Biggest Clock face
- 24 hour analog clocks
- Science Museum - more details on early clocks
- Humanclock.com a website with a unique image for every minute of the day
- Block clock
- [3] article, by a key figure in the development of quartz crystal clocks, on the history of timekeeping up to the late 1940s (based on remarks he made when given a prize for his contributions to horology) from The Bell System Technical Journal, Vol. XXVII, pp. 510-588, 1948
www.Timeforclocks.nl: information on Dutch clocks
Footnotes
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References
- Bruton, Eric. The History of Clocks and Watches. London: Black Cat, 1993.
- Edey, Winthrop. French Clocks. New York: Walker & Co., 1967.
- Lloyd, Alan H. “Mechanical Timekeepers.” In A History of Technology. Vol. III. Edited by Charles Joseph Singer, et. al. Oxford: Clarendon Press, 1957, pp. 648-675.
- Robinson, Tom. The Longcase Clock. Suffolk, England: Antique Collector’s Club, 1981.
- Smith, Alan. The International Dictionary of Clocks. London: Chancellor Press, 1996.
- Tardy. French Clocks the World Over. Part One and Two. Translated with the assistance of Alexander Ballantyne. Paris: Tardy, 1981.
- Yoder, Joella Gerstmeyer. Unrolling Time: Christiaan Huygens and the mathematization of nature. New York: Cambridge University Press, 1988.bg:Часовник
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