Clock
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Reverted edits by [[Special:Contributions/71.63.29.1|71.63.29.1]] ([[User talk:71.63.29.1|talk]]) to last version by Epbr123
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[[Image:Clock in Kings Cross.jpg|thumb|right|220px|Platform clock at [[London King's Cross railway station|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 language|Dutch]], Northern French, and [[Medieval Latin]]) from the [[Celtic languages|Celtic]] words ''clagan'' and ''clocca'' meaning "bell". For [[Horology|horologists]] and other specialists the term ''clock'' continues to mean exclusively a device with a [[Striking clock|striking mechanism]] for announcing intervals of time acoustically, by ringing a bell, a set of [[Chime (bell instrument)|chimes]], or a [[gong]].{{Fact|date=September 2007|few, if any, dictionaries have issue using "clock" to refer to silent clocks - such as atomic clocks}} A silent instrument lacking such a mechanism has traditionally been known as a '''timepiece'''.<ref>see Baillie et al., p. 307; Palmer, p. 19; Zea & Cheney, p. 172</ref> In general usage today, however, a "clock" refers to any device for measuring and displaying the time which, unlike a [[watch]], is not worn on the person.
[[Image:Greenwich clock.jpg|thumb|right|220px|Clock at the Royal Observatory, Greenwich]]
==History==
{{See|History of timekeeping devices}}
[[Image:IncenseAlarmClock.JPG|thumb|right|Replica of an ancient [[China|Chinese]] [[incense clock]]]]
The clock is one of the oldest human [[invention]]s, 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 operating on several different physical processes have been used over the millennia, culminating in the clocks of today.
===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 history|ancient times]]. A well-designed sundial can measure local [[solar time]] with reasonable accuracy, and sundials continued 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 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.
===Water clocks===
{{main|Water clock}}
[[Image:SuSongClock1.JPG|thumb|right|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 [[gnomon]] and the day-counting [[tally stick]].<ref>{{Harvnb|Turner|1984|p=1}}</ref> 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]], 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.<ref>{{Harvnb|Cowan|1958|p=58}}</ref>
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 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.
===Early clocks===
In 797 (or possibly 801), the [[Abbasid]] [[caliph]] of [[Baghdad]], [[Harun al-Rashid]], presented [[Janae]] with an [[Asian Elephant]] named [[Abul-Abbas]] together with a "particularly elaborate example" of a water <ref>{{cite book | last = James | first = Peter | title = Ancient Inventions | year = 1995 | location = New York, NY | id = ISBN 0-345-40102-6 | page = 126}}</ref> clock.
None of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.
Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be strictly regulated. This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination. Important times and durations were broadcast by bells, rung either by hand or by some mechanical device such as a falling weight or rotating beater.
The word ''horologia'' (from the Greek ὡρα, hour, and λεγειν, to tell) was used to describe all these devices, but the use of this word (still used in several [[romance languages]]) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 [[Cathédrale Saint-Étienne de Sens|Sens Cathedral]] installed a ‘horologe’ but the mechanism used is unknown. According to [[Jocelin of Brakelond]], in 1198 during a fire at the abbey of St Edmundsbury (now [[Bury St Edmunds]]), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire <ref>{{cite book | title=The Chronicle of Jocelin of Brakelond, Monk of St. Edmundsbury: A Picture of Monastic and Social Life on the XIIth Century | year=1907 | location=London | publisher=Chatto and Windus. Translated and edited by L. C. Jane}}</ref>.
These early clocks may not have used hands or dials, but “told” the time with audible signals.
====A new mechanism====
The word ''clock'' (from the Latin word ''clocca'', "bell"), which gradually supersedes "horologe", suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century in Europe.
Between 1280 and 1320, there is an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used [[Hydropower|water power]] were being adapted to take their driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power - the [[escapement]] - marks the beginning of the true mechanical clock.
Outside of Europe, the escapement mechanism had been known and used in medieval China, as the [[Song Dynasty]] horologist and engineer [[Su Song]] (1020 - 1101) incorporated it into his astronomical clock-tower of Kaifeng in 1088<ref>History of Song 宋史, Vol. 340</ref>. However, his astronomical clock and rotating [[armillary sphere]] still relied on the use of flowing water (ie. [[hydraulics]]), while European clockworks of the following centuries shed this old habit for a more efficient driving power of weights, in addition to the escapement mechanism.
The first mechanical clocks to be driven by [[Maintaining power|weights]] and [[gear]]s were invented by medieval [[Timeline of Muslim scientists and engineers|Muslim engineers]].<ref name=Salim>Professor [[Salim Al-Hassani]] (2006), ''1001 Inventions: Muslim Heritage in Our World'', FSTC, ISBN 0955242606</ref><ref name="Where the heart is">[http://www.1001inventions.com/index.cfm?fuseaction=main.viewSection&intSectionID=240 Where the heart is], ''1001 Inventions: Muslim Heritage in Our World'', 2006</ref> The first geared mechanical clock was invented by the 11th-century [[Arab]] engineer Ibn Khalaf al-Muradi in [[Al-Andalus|Islamic Spain]]; the first [[Maintaining power|weight-driven]] mechanical clocks, employing a mercury escapement mechanism and a [[clock face]] similar to an [[astrolabe]] dial, were also invented by Muslim engineers in the 11th century. A similar weight-driven mechanical clock later appeared in a [[Spanish language]] work compiled from earlier Arabic sources for [[Alfonso X]] in 1277.<ref name=Hassan>[[Ahmad Y Hassan]], [http://www.history-science-technology.com/Articles/articles%2071.htm Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering], ''History of Science and Technology in Islam''.</ref> The knowledge of weight-driven mechanical clocks produced by Muslim engineers in Spain was transmitted to other parts of Europe through [[Latin translations of the 12th century|Latin translations]] of Arabic and Spanish texts on [[Inventions in the Islamic world|Muslim mechanical technology]].<ref name=Ajram>{{Harvard reference |last=Ajram |first=K. |year=1992 |title=Miracle of Islamic Science |chapter=Appendix B |publisher=Knowledge House Publishers |isbn=0911119434 }}</ref>
[[Image:Al-jazari elephant clock.png|thumb|right|200px|An [[elephant clock]] in a manuscript by [[Al-Jazari]] (1206 AD) from ''The Book of Knowledge of Ingenious Mechanical Devices''. <ref>[[Ibn al-Razzaz Al-Jazari]] (ed. 1974), ''The Book of Knowledge of Ingenious Mechanical Devices''. Translated and annotated by [[Donald Routledge Hill]], Dordrecht/D. Reidel.</ref>]]In the 13th century, clock construction and engineering entered a new phase with the advancements made by [[Al-Jazari]], a Muslim engineer from Diyar-Bakr in South East Turkey, who is thought to be behind the birth to the concept of automatic machines{{Fact|date=October 2007}}. While working for [[Artuqid]] king of Diyar-Bakr, [[Nasir al-Din]], al-Jazari made numerous clocks of all shapes and sizes. In 1206 he was ordered by the king to document his inventions leading to the publication of an outstanding book on engineering called "The Book of Knowledge of Ingenious Mechanical Devices” {{Fact|date=October 2007}}. This book became an invaluable resource for people of different engineering backgrounds as it described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included the Elephant, the Castle and Scribe clocks, all of which were reconstructed by Muslim Heritage Consulting for Ibn Battuta Shopping Mall in Dubai (UAE), where they are fully functional. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.<ref>al-Hassani, Woodcok and Saoud (2007), 'Muslim Heritage in Our World', FSTC publishing pp.14-17</ref>
These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modeling the [[solar system]]. The former purpose is administrative, the latter arises naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The [[astrolabe]] was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system.
Simple clocks intended mainly for notification were installed in towers, and did not always require dials or hands. They would have announced the [[canonical hours]] or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including [[hour|Italian hours]], canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata.
In 1283, a large clock was installed at [[Dunstable Priory]]; its location above the [[rood screen]] suggests that it was not a water clock {{Fact|date=October 2007}}. In 1292, [[Canterbury Cathedral]] installed a 'great horloge'. Over the next 30 years there are brief mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a new clock was installed in [[Norwich]], an expensive replacement for an earlier clock installed in 1273. This had a large (2 metre) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two [[clockkeeper]]s for two years {{Fact|date=October 2007}}.
====Early astronomical clocks====
[[Image:Abbot_Richard_Wallingford.jpg|thumb|right|[[Richard of Wallingford]] pointing to a clock, his gift to [[St Albans Abbey]]]]
Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed by [[Richard of Wallingford]] in [[St Albans]] by 1336, and by Giovanni [[de Dondi]] in [[Padua, Italy|Padua]] from 1348 to 1364. They no longer exist, but detailed descriptions of their design and construction survive,{{Fact|date=October 2007}} while modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena.
Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a wheel of fortune and an indicator of the state of the tide at [[London Bridge]]. Bells rang every hour, the number of strokes indicating the time.
Dondi's clock was a seven-sided construction, 1 metre high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and [[Moveable feast|movable feasts]], and an eclipse prediction hand rotating once every 18 years.
It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture.
The [[Salisbury cathedral clock|Salisbury Cathedral clock]], built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours<ref>Singer, Charles, et al ''Oxford History of Technology: volume II, from the Renaissance to the Industrial Revolution'' (OUP 1957)pg 650-1</ref>.
===Later developments===
Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried.
Spring-driven clocks appeared during the 1400s,<ref>{{Cite book
| last=Usher
| first=Abbot Payson
| title=A History of Mechanical Inventions
| date=1988 | publisher=Courier Dover
| isbn=048625593X
| url=http://books.google.com/books?id=xuDDqqa8FlwC&pg=PA305&sig=_SRpwfz0YBAjt2aGxXhmRkZ16GQ}}, p.305</ref><ref name="White">{{cite book
| last=White
| first=Lynn Jr.
| title=Medieval Technology and Social Change
| publisher=Oxford Univ. Press
| date=1966
| location=New York
| isbn=0195002660 }}, p.126-127</ref><ref name="Rossum">{{cite book
| last=Dohrn-van Rossum
| first=Gerhard
| title=History of the Hour: Clocks and Modern Temporal Orders
| publisher=Univ. of Chicago Press
| date=1997
| url=http://books.google.com/books?id=53K32RiEigMC&pg=PA121&sig=5huN81ukYRbSlxq4MsToTDIXYDY
| isbn=0-226-15510-2 }} p.121</ref> although they are often erroneously credited to [[Nürnberg]] watchmaker [[Peter Henlein]] (or Henle, or Hele) around 1511.<ref>{{cite book
| last=Milham
| first=Willis I.
| title=Time and Timekeepers
| date=1945
| publisher=MacMillan
| location=New York
| isbn=0780800087}}, p.121</ref><ref>{{ cite encyclopedia
| title=Clock
| encyclopedia=The New Encyclopaedia Britannica
| volume=4
| page=747
| publisher=Univ. of Chicago
| date=1974
| url=http://books.google.com/books?as_brr=0&id=Eb0qAAAAMAAJ&dq=Peter+Henlein+mainspring&q=peter+Henlein&pgis=1#search
| isbn=0852292902}}</ref><ref>{{cite book
| last=Anzovin
| first=Steve
| coauthors=Podell, Janet
| title=Famous First Facts: A record of first happenings, discoveries, and inventions in world history
| date=2000
| publisher=H.W. Wilson
| url=
| isbn=0824209583}}, p.440</ref> The earliest existing spring driven clock is the chamber clock given to Peter the Good, Duke of Burgundy, around 1430, now in the [[Germanisches Nationalmuseum]].<ref name="White" /> Spring power presented clockmakers with a new problem; how to keep the clock movement running at a constant rate as the spring ran down. This resulted in the invention of the ''stackfreed'' and the [[fusee]] in the 1400s, and many other innovations, down to the invention of the modern ''going [[barrel (horology)|barrel]]'' in 1760.
The first record of a minute hand on a clock is 1475, in the Almanus Manuscript of Brother Paul {{Fact|date=October 2007}}.
During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of [[Nuremberg]] and [[Augsburg]], and in France, [[Blois]]. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The [[cross-beat escapement]] {{Fact|date=October 2007}} was developed in 1585 by [[Jost Burgi]], who also developed the [[remontoire]]. Burgi's accurate clocks helped [[Tycho Brahe]] to observe astronomical events with much greater precision than before.
The first mechanical [[alarm clock]] was invented by the [[Ottoman Empire|Ottoman]] engineer [[Taqi al-Din]]. He described the alarm clock in his book, ''The Brightest Stars for the Construction of Mechanical Clocks'' (''Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya''), published in 1556-1559. His alarm clock was capable of sounding at a specified time, achieved by placing a peg on the dial wheel. At the requested time, the peg activated a ringing device.<ref>{{cite web|author=[[Salim Al-Hassani]]|title=The Astronomical Clock of Taqi Al-Din: Virtual Reconstruction|publisher=FSTC|url=http://muslimheritage.com/topics/default.cfm?ArticleID=947|date=19 June 2008|accessdate=2008-07-02}}</ref> In the same treatise, he described a mechanical [[astronomical clock]] called the "observational clock", which was the first to measure time in [[minute]]s. He made use of his [[Islamic mathematics|mathematical knowledge]] to design three [[Clock face|dials]] which showed the hours, degrees and minutes.<ref name=Hassani>{{cite web|author=Munim M. Al-Rawi and [[Salim Al-Hassani]]|title=The Contribution of Ibn Sina (Avicenna) to the development of Earth sciences|publisher=FSTC|url=http://www.muslimheritage.com/uploads/ibnsina.pdf|date=November 2002|accessdate=2008-07-01}}</ref> He later improved the design of his observational clock to measure time in [[second]]s in an [[Islamic astronomy|astronomical treatise]] written at his [[Istanbul observatory of al-Din]] (1577-1580). He described his observational clock as "a mechanical clock with three dials which show the hours, the minutes, and the seconds." This was an important innovation in 16th-century practical astronomy, as previous clocks were not accurate enough to be used for astronomical purposes.<ref name=Tekeli>{{cite encyclopedia | first = Sevim | last = Tekeli | title = Taqi al-Din | year = 1997 | encyclopedia = Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures | publisher = [[Kluwer Academic Publishers]] | ISBN = 0792340663 | url = http://www.springer.com/philosophy/philosophy+of+sciences/book/978-1-4020-4425-0 }}</ref> He further improved the observational clock, using only one dial to represent the hours, minutes and seconds, describing it as "a mechanical clock with a dial showing the hours, minutes and seconds and we divided every minute into five seconds."<ref>{{citation|first=Aydin|last=Sayili|authorlink=Aydin Sayili|title=The Observatory in Islam|year=1991|pages=289-305}} ([[cf.]] {{cite web|author=Dr. Salim Ayduz|title=Taqi al-Din Ibn Ma’ruf: A Bio-Bibliographical Essay|url=http://muslimheritage.com/topics/default.cfm?ArticleID=949|date=26 June 2008|accessdate=2008-07-04}})</ref>
Another early record of a second hand on a clock dates back to about 1560, on a clock now in the [[Fremersdorf]] collection.{{Fact|date=October 2007}} However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working.
[[Image:Musée du temps Besançon 3.jpg|thumb|French [[rococo]] bracket clocks, (Museum of Time, Besançon)]]
The next development in accuracy occurred after 1657 with the invention of the [[pendulum clock]]. [[Galileo Galilei|Galileo]] had the idea to use a swinging bob to regulate 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]],{{Fact|date=October 2007}} an improvement over Huygens' [[crown escapement]] {{Fact|date=October 2007}}. Within just one generation, [[minute]] hands and then [[second]] hands were added.
A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, which would be virtually useless on a rocking ship. Many European governments offered a large [[longitude prize|prize]] for anyone that could determine longitude accurately; for example, Great Britain offered 20,000 pounds, equivalent to millions of dollars today. The reward was eventually claimed in 1761 by [[John Harrison]], who dedicated his life to improving the accuracy of his clocks. His [[John Harrison#The longitude watches|H5]] clock is reported to have lost less than 5 seconds over 10 days.{{Fact|date=October 2007}}
The excitement over the pendulum clock had 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]] is also 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 face]]s to utilize [[Vitreous enamel|enamel]] as well as hand-painted ceramics.[[Image:Horloge-republicaine1.jpg|thumb|right|French decimal clock from the time of the [[French Revolution]]]]
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.
[[Alexander Bain (inventor)|Alexander Bain]], Scottish clockmaker, patented the [[electric clock]] in 1840. The electric clock's mainspring is wound either with an [[electric motor]] or with an [[electro-magnet]] and armature. In 1841, he first patented the [[electromagnetic]] pendulum.
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 vibration of a [[tuning fork]], the behaviour of [[quartz]] crystals, the resonance of polycarbonates.{{Fact|date=October 2007}}, or the quantum vibrations of atoms. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.
==How clocks work==
The invention of the mechanical clock in the 13th century started a change in timekeeping methods from [[Continuous function|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.<ref name="Cipolla">{{cite book
|last=Cipolla
|first=Carlo M.
|title=Clocks and Culture, 1300 to 1700
|date=2004
|publisher=W.W. Norton & Co.
|isbn=0393324435|url=http://books.google.com/books?id=YSf9MVxa2JEC&pg=PA31&dq=verge+escapement+technology&sig=6ZbQh-an59yCcesR1mjn1p8w-H4}}, p.31</ref> 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.<ref name="Jespersen">{{cite book
| last = Jespersen
| first = James
| authorlink =
| coauthors = Fitz-Randolph, Jane; Robb, John
| title = From Sundials to Atomic Clocks: Understanding Time and Frequency
| publisher = Courier Dover
| date = 1999
| location = New York
| pages =
| url = http://books.google.com/books?id=Z7chuo4ebUAC&pg=PA42&dq=clock+resonance+pendulum&lr=&sig=iBunChocEtJoeKS5p5IgJ1oyl4U
| doi =
| id =
| isbn =0486409139 }} p.39</ref><ref>{{cite web
| title = How clocks work
| work = InDepthInfo
| publisher = W. J. Rayment
| date = 2007
| url = http://www.indepthinfo.com/clocks/index.shtml
| format =
| doi =
| accessdate = 2008-06-04}}</ref><ref name="Milham">{{cite book
| last = Milham
| first = Willis I.
| authorlink =
| coauthors =
| title = Time and Timekeepers
| publisher = MacMillan
| date = 1945
| location = New York
| pages =
| url =
| doi =
| id =
| isbn = 0780800087}} p.74</ref> 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 ''counter''s 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.
===Power source===
This provides power to keep the clock going.
*In [[mechanical clock]]s, this is either a weight suspended from a cord wrapped around a [[pulley]], or a spiral [[spring]] called a [[mainspring]].
*In [[electric clock]]s, it is either a [[Battery (electricity)|battery]] or the [[Mains power|AC power line]].
Since clocks must run continuously, there is often a small secondary power source to keep the clock going temporarily during interruptions in the main power. In old mechanical clocks, a ''[[maintaining power]] spring'' provided force to turn the clock's wheels while the mainspring was being wound up.<ref>{{cite book
| last = Milham
| first = Willis I.
| authorlink =
| coauthors =
| title = Time and Timekeepers
| publisher = MacMillan
| date = 1945
| location = New York
| pages =
| url =
| doi =
| id =
| isbn = 0780800087}} p.174-175</ref> In [[quartz clock]]s a [[backup battery]] or [[capacitor]] is often included to keep the clock going if the power cord is unplugged.
===Oscillator===
The timekeeping element in every modern clock is a [[harmonic oscillator]], a physical object ([[resonator]]) that vibrates or [[oscillate]]s repetitively at a precisely constant [[frequency]].<ref name="Marrison">{{cite journal
| last = Marrison
| first = Warren
| authorlink =
| coauthors =
| title = The Evolution of the Quartz Crystal Clock
| journal = Bell System Technical Journal
| volume = 27
| issue =
| pages = 510–588
| publisher = American Telephone and Telegraph Co.
| location =
| date = 1948
| url = http://www.ieee-uffc.org/fcmain.asp?page=marrison
| doi =
| id =
| accessdate = 2008-06-04}}</ref>
*In mechanical clocks, this is either a [[pendulum]] or a [[balance wheel]].
*In some early electronic clocks and watches such as the [[Accutron]], it is a [[tuning fork]].
*In [[quartz clock]]s and watches, it is a [[quartz crystal]].
*In [[atomic clock]]s, it is the vibration of [[electron]]s in [[atom]]s as they emit [[microwave]]s.
*In early mechanical clocks before 1657, it was a crude balance wheel or [[foliot]] which was not a harmonic oscillator because it lacked a [[balance spring]]. As a result they were very inaccurate, with errors of perhaps an hour a day.<ref>Milham, 1945, p.85</ref>
The advantage of a harmonic oscillator over other forms of oscillator is that it employs [[resonance]] to vibrate at a precise natural [[resonant frequency]] or 'beat' dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its [[Q_factor|Q]],<ref>{{cite web
| title = Quality factor, Q
| work = Glossary
| publisher = Time and Frequency Division, NIST (National Institute of Standards and Technology)
| date = 2008
| url = http://tf.nist.gov/general/enc-q.htm
| format =
| doi =
| accessdate = 2008-06-04}}</ref><ref>[http://books.google.com/books?id=Z7chuo4ebUAC&pg=PA44&sig=iBunChocEtJoeKS5p5IgJ1oyl4U Jespersen 1999, p.47-50]</ref> or quality factor, which increases (other things being equal) with its resonant frequency.<ref>{{cite book
| last = Riehle
| first = Fritz
| authorlink =
| coauthors =
| title = Frequency Standards: Basics and Applications
| publisher = Wiley VCH Verlag & Co.
| date = 2004
| location = Germany
| pages =
| url = http://books.google.com/books?id=WZ34pQV-DXMC&pg=PA9&dq=Q+linewidth+%22split+the+line%22&lr=&as_brr=3&sig=dUBX3lf0vjScZANZAJzNc8C7uoc
| doi =
| id =
| isbn = 3527402306}} p.9</ref> This is why there has been a long term trend toward higher frequency oscillators in clocks. Balance wheels and pendulums always include a means of adjusting the rate of the timepiece. Quartz timepieces sometimes include a rate screw that adjusts a [[capacitor]] for that purpose. Atomic clocks are [[Standard (technical)|primary standards]], and their rate cannot be adjusted.
====Synchronized or slave clocks====
Some clocks rely for their accuracy on an external oscillator; that is, they are automatically [[Synchronization|synchronized]] to a more accurate clock:
*[[Slave clock]]s, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a [[master clock]] in the building, and periodically received a signal to synchronize them with the master, often on the hour.<ref>Milham, 1945, p.325-328</ref>
*[[Electric clock|Synchronous electric clocks]] don't have an internal oscillator, but rely on the 50 or 60 [[Hz]] oscillation of the [[Mains power|AC power line]], which is synchronized by the utility to a precision oscillator. This drives a [[synchronous motor]] in the clock which rotates once for every cycle of the line voltage, and drives the gear train.
*Computer [[real time clock]]s keep time with a quartz crystal, but are periodically (usually weekly) synchronized over the internet to atomic clocks ([[UTC]]), using a system called [[Network Time Protocol]].
*[[Radio clock]]s keep time with a quartz crystal, but are periodically (often daily) synchronized to atomic clocks ([[UTC]]) with time signals from government radio stations like [[WWV]], [[WWVB]], [[CHU]], [[DCF77]] and the [[GPS]] system.
===Controller===
This has the dual function of keeping the oscillator running by giving it 'pushes' to replace the energy lost to [[friction]], and converting its vibrations into a series of pulses that serve to measure the time.
*In mechanical clocks, this is the [[escapement]], which gives precise pushes to the swinging pendulum or balance wheel, and releases one gear tooth of the ''escape wheel'' at each swing, allowing all the clocks wheels to move forward a fixed amount with each swing.
*In electronic clocks this is an [[Electronic oscillator|electronic oscillator circuit]] that gives the vibrating quartz crystal or tuning fork tiny 'pushes', and generates a series of electrical pulses, one for each vibration of the oscillator, which is called the [[clock signal]].
*In [[atomic clock]]s the controller is an evacuated [[microwave]] [[Cavity resonator|cavity]] attached to a microwave [[Electronic oscillator|oscillator]] controlled by a [[microprocessor]]. A thin gas of [[cesium]] atoms is released into the cavity where they are exposed to [[microwaves]]. A laser measures how many atoms have absorbed the microwaves, and an electronic [[feedback]] control system called a [[phase locked loop]] tunes the microwave oscillator until it is at the exact frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by [[digital counter]]s to become the [[clock signal]].<ref> [http://books.google.com/books?id=Z7chuo4ebUAC&pg=PA61&sig=r7PLMbI4rhAgfGkfBS-MCJEBkVs Jespersen 1999, p.52-62]</ref>
In mechanical clocks, the low [[Q_factor|Q]] of the balance wheel or pendulum oscillator made them very sensitive to the disturbing effect of the impulses of the escapement, so the escapement had a great effect on the accuracy of the clock, and many escapement designs were tried. The higher Q of resonators in electronic clocks makes them relatively insensitive to the disturbing effects of the drive power, so the driving oscillator circuit is a much less critical component.<ref name="Marrison" />
===Counter chain===
This counts the pulses and adds them up to get traditional time units of [[second]]s, [[minute]]s, [[hour]]s, etc. It usually has a provision for ''setting'' the clock by manually entering the correct time into the counter.
*In mechanical clocks this is done [[Analog computer|analogically]] by a [[gear train]], known as the [[wheel train (horology)|wheel train]]. The gear train also has a second function; to transmit mechanical power from the power source to run the oscillator. There is a friction coupling called the 'cannon pinion' between the gears driving the hands and the rest of the clock, allowing the hands to be turned by a knob on the back to set the time.<ref>Milham, 1945, p.113</ref>
*In digital clocks a series of [[integrated circuit]] [[counter]]s or dividers add the pulses up [[digital]]ly, using [[binary]] logic. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time.
===Indicator===
This displays the count of seconds, minutes, hours, etc. in a human readable form.
*The earliest mechanical clocks in the 13th century didn't have a visual indicator and signalled the time [[Audible|audibly]] by striking [[bell]]s. Many clocks to this day are [[striking clock]]s which chime the hours.
*[[Analog clock]]s, including almost all mechanical and some electronic clocks, have a traditional dial or [[clock face]], that displays the time in [[analog]] form with moving hour and minute hand. In [[quartz clock]]s with analog faces, a 1 [[Hz]] signal from the counters actuates a [[stepper motor]] which moves the second hand forward at each pulse, and the minute and hour hands are moved by gears from the shaft of the second hand.
*[[Digital clock]]s display the time in periodically changing [[digit]]s on a digital [[display]].
*[[Talking clock]]s and the [[speaking clock]] services provided by telephone companies speak the time audibly, using either recorded or digitally [[Voice synthesis|synthesized voices]].
==Types==
Clocks can be classified by the type of time display, as well as by the method of timekeeping.
===Time display methods===
====Analogue clocks====
[[Image:Picadillycircuslinearclock.jpg|thumb|300px|right|A linear clock at [[London]]'s [[Piccadilly Circus tube station]]. The 24 hour band moves across the static map, keeping pace with the apparent movement of the sun above ground, and a pointer fixed on London points to the current time]]
Analogue clocks usually indicate time using angles. 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 [[hour]]s, which can also serve as a scale of 60 [[minute]]s, and 60 [[seconds]] if the clock has a second hand. Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. The only other widely used clock face today is the [[24 hour analogue dial]], because of the use of [[24 hour time]] in [[military]] organizations and timetables. The [[10-hour clock]] was briefly popular during the [[French Revolution]], when the [[metric system]] was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch chiming 24 times uses more power).
Another type of analogue clock is the [[sundial]], which tracks the sun continuously, registering the time by the shadow position of its [[gnomon]]. Sundials use some or part of the 24 hour analogue dial. There also exist clocks which use a digital display despite having an analogue mechanism—these are commonly referred to as flip clocks.
Alternative systems have been proposed. For example, the [[TWELV]] clock indicates the current hour using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a [[Lunar phase|moon phase]].
The mechanics of analogue clocks were also the subject of the Grammy Award winning Coldplay single, ''Clocks'' in which the continual ticking of the clocks mesmerises and fascinates the narrator of the song.
====Digital clocks====
[[Image:KanazawaStationClock.jpg|thumb|right|Digital clock outside [[Kanazawa Station]] displaying the time by controlling valves on a fountain]]
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]], [[LED]], or [[Vacuum fluorescent display|VFD]] display; many other display technologies are used as well ([[cathode ray tube]]s, [[nixie tube]]s, etc.). After a reset, battery change or power failure, digital clocks without a backup [[battery (electricity)|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 radio or Internet time servers that are tuned to national [[atomic clock]]s. Since the release of digital clocks in the mainstream, the use of analogue clocks has dropped dramatically.
[[Image:Digital-clock-radio-basic hf.jpg|thumb|Basic digital clock radio]]
====Auditory clocks====
{{main|Talking clock}}
For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. The sound 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 [[Clock Tower, Palace of Westminster|Big Ben]]). Most telecommunication companies also provide a [[Speaking clock]] service as well.
==Purposes==
Clocks are in homes, offices and many other places; smaller ones ([[watch]]es) are carried on the wrist; larger ones are in public places, e.g. a [[train station]] or [[church]]. A small clock is often shown in a corner of [[computer display]]s, [[mobile phone]]s and many [[MP3 player]]s.
The 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 (explosive)|time bomb]] (see: [[counter]]). However, in this context, it is more appropriate to refer to it as a [[timer]] or [[trigger mechanism]] rather than strictly as a clock.
[[Computer]]s depend on an accurate internal [[clock signal]] to allow synchronized processing. (A few research projects are developing CPUs based on [[asynchronous circuit]]s.) 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 internal computer clock is generally kept running by a small battery. 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.{{Fact|date=October 2007}} 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. An ideal clock should too measure time in consistent, for example decimalized time units.
This leads to the following definitions:
*A clock is a recurrent [[Process (general)|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 (e.g. 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 clock]]s 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.[[Image:Harrison's Chronometer H5.JPG|thumb|[[John Harrison]]'s Chronometer H5]]
===Navigation===
[[Navigation]] by ships 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 chronometer]] in the mid-18th century. The [[Noon gun]] in [[Cape Town]] still fires an accurate signal to allow ships to check their [[marine chronometer|chronometers]].
Use of a common clock in radio signal producing [[satellite]]s is fundamental to the operation of [[Global Positioning System|GPS]] (Global Positioning System) navigation devices.
==Seismology==
In determining the location of an [[earthquake]], the arrival time of several types of [[Seismic wave|seismic wave]] at at least four dispersed observers is dependent upon each observer recording wave arrival times according to a common clock.
==Specific types of clocks==
{{col-begin}}
{{col-3}}
* [[Alarm clock]]
* [[Analog clock with digital display|Flip clock]]
* [[Astronomical clock]]
* [[Atomic clock]]
* [[Balloon clock]]
* [[Binary clock]]
* [[Bracket clock]]
* [[Carriage clock]]
* [[Cartel clock]]
* [[Chiming clock]]
* [[Clock network]]
* [[Clock of the Long Now]]
* [[Clock tower]]
* [[Countdown clock]]
* [[Cuckoo clock]]
* [[Data]] clock for [[timescapes]] created with [[time-technology]]
* [[Digital clock]]
* [[Doll's head clock]]
* [[Electric clock]]
{{col-3}}
* [[Floral clock]]
* [[Game clock]]
* [[Hourglass]]
* [[Japanese clock]]
* [[Lantern clock]]
* [[Lighthouse Clock]]
* [[Longcase clock|Longcase (or "grandfather") clock]]
* [[Master clock]]
* [[Mantel clock]]
* [[Musical clock]]
* [[Paper clock]]
* [[Pedestal clock]]
* [[Pendulum clock]]
* [[Projection clock]]
* [[Quartz clock]]
* [[Radio clock]]
* [[Railroad chronometers|Railroad chronometer]]
* [[Reference clock]]
* [[Rolling ball clock]]
{{col-3}}
* [[Sidereal clock]]
* [[Skeleton clock]]
* [[Slave clock]]
* [[Speaking clock]]
* [[Stopwatch]]
* [[Striking clock]]
* [[Sundial]]
* [[Talking clock]]
* [[Longcase clock|Tall-case clock]]
* [[Tide clock]]
* [[Time ball]]
* [[Time clock]]
* [[Torsion pendulum clock]]
* [[Clock tower|Tower clock]]
* [[Wall clock]]
* [[Watch]]
* [[Water clock]]
* [[World clock]]
{{col-end}}
==See also==
{{col-begin}}
{{col-break}}
* [[Allan variance]]
* [[American Watchmakers-Clockmakers Institute]]
* [[BaselWorld]]
* [[Biological clock]]
* [[Lewis Mumford#The clock as herald of the Industrial Revolution|Clock as herald of the Industrial Revolution]] (Lewis Mumford)
* [[Clock face]]
* [[Clock of the Long Now]]
* [[Clockmaker]]
* [[Clock signal]] (digital circuits)
* [[Colgate Clock (New Jersey)]], the world's largest clock
* [[Cox's timepiece]]
* [[Death Clock]]
* [[Department of Defense master clock]] (U.S.)
{{col-break}}
* [[Doomsday Clock]]
* [[Earth clock]]
* [[Federation of the Swiss Watch Industry FH]]
* [[Guard tour patrol system]] (Watchclocks)
* [[Humanclock]]
* [[Iron Ring Clock]]
* [[Jens Olsen's World Clock]]
* [[Jewel bearing]]
* [[Metrology]]
* [[National Association of Watch and Clock Collectors]]
{{col-break}}
* [[Replica watch]]
* [[Star clock]]
* [[Steam clock]]
* [[System time]]
* [[Timeline of time measurement technology]]
* [[Timer]]
* [[Time to digital converter]]
* [[Watchmaker]]
{{col-end}}
==Notes==
{{reflist}}
==References==
* Baillie, G.H., O. Clutton, & C.A. Ilbert. ''Britten’s Old Clocks and Watches and Their Makers'' (7th ed.). Bonanza Books (1956).
* Bolter, David J. ''Turing's Man: Western Culture in the Computer Age''. The University of North Carolina Press, Chapel Hill, N.C. (1984). ISBN 0-8078-4108-0 pbk. Very good, readable summary of the role of "the clock" in its setting the direction of philosophic movement for the "Western World". Cf. picture on p. 25 showing the ''verge'' and ''foliot''. Bolton derived the picture from Macey, p. 20.
* Bruton, Eric. ''The History of Clocks and Watches''. London: Black Cat (1993).
*{{cite book | last = Dohrn-van Rossum | first = Gerhard | others = Trans. Thomas Dunlap | title = History of the Hour: Clocks and Modern Temporal Orders | year = 1996 | publisher = The University of Chicago Press | location = Chicago | id = ISBN 0226155102}}
* Edey, Winthrop. ''French Clocks''. New York: Walker & Co. (1967).
* Kak, Subhash, Ph.D. Babylonian and Indian Astronomy: Early Connections. February 17, 2003.
* Kumar, Narendra "Science in Ancient India" (2004). ISBN 8126120568.
* Landes, David S. ''Revolution in Time: Clocks and the Making of the Modern World''. Cambridge: Harvard University Press (1983).
* Lloyd, Alan H. “Mechanical Timekeepers”, ''A History of Technology,'' Vol. III. Edited by Charles Joseph Singer et al. Oxford: Clarendon Press (1957), pp. 648-675.
* Macey, Samuel L., ''Clocks and the Cosmos: Time in Western Life and Thought'', Archon Books, Hamden, Conn. (1980).
*{{cite book | last = Needham | first = Joseph | authorlink = Joseph Needham | title = Science & Civilisation in China, Vol. 4, Part 2: Mechanical Engineering | origyear = 1965 | year = 2000 | publisher = Cambridge University Press | location = Cambridge | id = ISBN 0521058031}}
* North, John. ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. London: Hambledon and London (2005).
* Palmer, Brooks. ''The Book of American Clocks'', The Macmillan Co. (1979).
* 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 I and II. 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).
* Zea, Philip, & Robert Cheney. ''Clock Making in New England – 1725-1825''. Old Sturbridge Village (1992).
==External links==
{{commonscat|Clocks}}
* [http://www.ieee-uffc.org/freqcontrol/marrison/Marrison.html Article, by a key figure in the development of quartz crystal clocks, on the history of timekeeping up to the late 1940s from ''The Bell System Technical Journal, Vol. XXVII, pp. 510-588, 1948'']
* [http://www.Timeforclocks.nl Information on Dutch clocks]
* [http://www.sciencemuseum.org.uk/visitmuseum/galleries/time_measurement.aspx Science Museum - Time Measurement]
* [http://www.spunk.org/texts/writers/woodcock/sp001734.html Tyranny of the Clock] - Brief analysis and history of the clock within the context of the industrial revolution
{{Time Topics}}
{{Time measurement and standards}}
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