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| {{multiple image
| | Compared to an array of rogue security programs circulating online, program security virus is very annoying. It's stubborn. It makes Windows unusable. Owners of infected machines would scream in despair. Assist on forums? Unsuspicious users can enumarate a dozen of recovery procedures that do work, nevertheless not with system security.<br><br>Although several online sites still provide free music downloads, nearly all of these music downloads are littered by spyware and virus. These websites are illegal plus can wreck havoc to we computer. You will choose to download music from these websites however, do at a peril. After being reincarnated Napster is today a safe plus legal music download website that provide free membership. Restrictions have been placed on some songs however, these are meant to safeguard the rights of the music firms plus the singers who created the song.<br><br>Like its Windows counterpart, BitDefender [http://leadingpcsoftware.com/best-antivirus-software/ what is the best antivirus software] best software for Mac is strong, flexible plus simple to use. It is totally capable of shielding the Mac OS X from all types of malware threats.<br><br>Cons: We should be connected to the web at all times to have the most current definitions. Best used on a desktop or a laptop which always has an internet connection Additionally if a malicious system is detected (whether or not it happens to be a false alarm) which system is blocked.<br><br>There are certain aspects I look for whenever I rate anti-virus programs. The apparent one is, does it block threats from infecting my computer. In many instances all "accredited" appear to function. Where I find a big difference is inside the small less noticeable things. I have detailed under a short list of factors why Norton is the number one choice.<br><br>We are here now to debunk all of the negative thoughts regarding Windows 7 protection. Being one of Microsoft's many stable performing operating systems, Windows 7 delivers a huge selection of security software.<br><br>Having an anti-virus system added to your computer is a smart choice. In life everything gets protected; homes, cars plus workplace spaces. So not safeguarding your computer while we surf the net is foolish. Utilizing the web securely plus feeling advantageous regarding it entails ordering a software system and adding it to the computer. For any questions we can constantly access their free support 24 hour contact numbers, so there is always assist in front of you should you want it. With all of the factors you do online, from banking to loading personal files plus photos, guaranteeing which information is safe is crucial. |
| | image1 = Galileo Pendulum Clock.jpg
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| | caption1 = Pendulum clock conceived by [[Galileo Galilei]] around 1637. The earliest known pendulum clock design, it was never completed.
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| | width1 = 138
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| | image2 = GB-3-Gew-Pendeluhr (Luekk).jpg
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| | caption2 = Vienna regulator style pendulum wall clock
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| | width2 = 110
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| }}
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| A '''pendulum clock''' is a [[clock]] that uses a [[pendulum]], a swinging weight, as its [[time]]keeping element. The advantage of a pendulum for timekeeping is that it is a [[harmonic oscillator]]; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates. From its invention in 1656 by [[Christiaan Huygens]] until the 1930s, the pendulum clock was the world's most precise timekeeper, accounting for its widespread use.<ref>{{cite book
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| |last=Milham
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| |first=Willis I.
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| |title=Time and Timekeepers
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| |year=1945
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| |publisher=MacMillan
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| |location=New York
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| |isbn=0-7808-0008-7}}, p.330, 334</ref><ref>{{cite journal
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| |last=Marrison
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| |first=Warren
| |
| |title=The Evolution of the Quartz Crystal Clock
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| |journal=Bell System Technical Journal
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| |year=1948
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| |volume=27
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| |pages=510–588
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| |url=http://www.ieee-uffc.org/freqcontrol/marrison/Marrison.html}}</ref> Pendulum clocks must be stationary to operate; any motion or [[acceleration]]s will affect the motion of the pendulum, causing inaccuracies, so other mechanisms must be used in portable timepieces. They are now kept mostly for their decorative and [[antique]] value.
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| | |
| ==History==
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| {{multiple image
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| | footer = The first pendulum clock, invented by Christiaan Huygens in 1656
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| | image1 = Huygens first pendulum clock - front view.png
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| | width1 = 120
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| | image2 = Huygens first pendulum clock.png
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| | width2 = 112
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| }}
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| The pendulum clock was invented in 1656 by [[Dutch (ethnic group and nation)|Dutch]] scientist [[Christiaan Huygens]], and patented the following year. Huygens contracted the construction of his clock designs to clockmaker [[Salomon Coster]], who actually built the clock. Huygens was inspired by investigations of pendulums by [[Galileo Galilei]] beginning around 1602. Galileo discovered the key property that makes pendulums useful timekeepers: [[wikt:isochronous|isochronism]], which means that the [[Frequency|period]] of swing of a pendulum is approximately the same for different sized swings.<ref>{{cite web
| |
| |title=Huygens' Clocks
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| |work=Stories
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| |publisher=Science Museum, London, UK
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| |url=http://www.sciencemuseum.org.uk/onlinestuff/stories/huygens_clocks.aspx
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| |accessdate=2007-11-14}}</ref><ref>{{cite web
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| |title=Pendulum Clock
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| |work=The Galileo Project
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| |publisher=Rice Univ.
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| |url=http://galileo.rice.edu/sci/instruments/pendulum.html
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| |accessdate=2007-12-03}}</ref> Galileo had the idea for a pendulum clock in 1637, which was partly constructed by his son in 1649, but neither lived to finish it.<ref>A modern reconstruction can be seen at {{cite web
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| |title=Pendulum clock designed by Galileo, Item #1883-29
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| |work=Time Measurement
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| |publisher=Science Museum, London, UK|url=http://www.sciencemuseum.org.uk/objects/time_measurement/1883-29.aspx
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| |accessdate=2007-11-14}}</ref> The introduction of the pendulum, the first [[harmonic oscillator]] used in timekeeping, increased the accuracy of clocks enormously, from about 15 minutes per day to 15 seconds per day<ref>{{cite web
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| |last=Bennet
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| |first=Matthew
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| |coauthors=et al.
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| |title=Huygens' Clocks
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| |year=2002
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| |publisher=Georgia Institute of Technology
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| |url=http://www.physics.gatech.edu/research/schatz/pubs/royclocks2.pdf
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| |format=PDF|accessdate=2007-12-04 |archiveurl = http://web.archive.org/web/20080410084732/http://www.physics.gatech.edu/research/schatz/pubs/royclocks2.pdf <!-- Bot retrieved archive --> |archivedate = 2008-04-10}}, p.3, also published in ''Proceedings of the Royal Society of London, ''A'' '''458''', 563–579</ref> leading to their rapid spread as existing '[[verge and foliot]]' clocks were retrofitted with pendulums.
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| [[Image:Edward East winged lantern clock.jpg|thumb|left|upright|A [[lantern clock]] that has been converted to use a pendulum. To accommodate the wide pendulum swings caused by the [[verge escapement]], "wings" have been added on the sides]]
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| These early clocks, due to their [[verge escapement]]s, had wide pendulum swings of up to 100°. In his 1673 analysis of pendulums, ''Horologium Oscillatorium'', Huygens showed that wide swings made the pendulum inaccurate, causing its [[Frequency|period]], and thus the rate of the clock, to vary with unavoidable variations in the driving force provided by the [[movement (clockwork)|movement]]. Clockmakers' realization that only pendulums with small swings of a few degrees are [[isochronous (horology)|isochronous]] motivated the invention of the [[anchor escapement]] around 1670, which reduced the pendulum's swing to 4°-6°.<ref>{{cite journal | |
| | last=Headrick
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| | first=Michael
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| | year=2002
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| | title=Origin and Evolution of the Anchor Clock Escapement
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| | journal=Control Systems magazine,
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| | publisher=Inst. of Electrical and Electronic Engineers
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| | volume=22
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| | issue=2
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| | url=http://www.webcitation.org/query?url=http://www.geocities.com/mvhw/anchor.html&date=2009-10-26+02:27:32
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| | accessdate=2007-06-06|archiveurl=http://web.archive.org/web/20091025120920/http://geocities.com/mvhw/anchor.html|archivedate=2009-10-25}}</ref> The anchor became the standard escapement used in pendulum clocks. In addition to increased accuracy, the anchor's narrow pendulum swing allowed the clock's case to accommodate longer, slower pendulums, which needed less power and caused less wear on the movement. The [[seconds pendulum]] (also called the Royal pendulum), 0.994 m (39.1 in) long, in which each swing takes one second, became widely used in quality clocks. The long narrow clocks built around these pendulums, first made by William Clement around 1680, became known as [[Longcase clock|grandfather clock]]s. The increased accuracy resulting from these developments caused the minute hand, previously rare, to be added to clock faces beginning around 1690.<ref>Milham 1945, p.190</ref>
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| | |
| The 18th and 19th century wave of [[horological]] innovation that followed the invention of the pendulum brought many improvements to pendulum clocks. The [[deadbeat escapement]] invented in 1675 by Richard Towneley and popularized by [[George Graham (clockmaker)|George Graham]] around 1715 in his precision "regulator" clocks gradually replaced the anchor escapement<ref>Milham 1945, p.181, 441</ref> and is now used in most modern pendulum clocks. Observation that pendulum clocks slowed down in summer brought the realization that [[thermal expansion]] and contraction of the pendulum rod with changes in temperature was a source of error. This was solved by the invention of temperature-compensated pendulums; the [[mercury pendulum]] by [[George Graham (clockmaker)|George Graham]] in 1721 and the [[gridiron pendulum]] by [[John Harrison]] in 1726.<ref>Milham 1945, p.193–195</ref> With these improvements, by the mid-18th century precision pendulum clocks achieved accuracies of a few seconds per week.
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| Until the 19th century, clocks were handmade by individual craftsmen and were very expensive. The rich ornamentation of pendulum clocks of this period indicates their value as status symbols of the wealthy. The [[clockmaker]]s of each country and region in Europe developed their own distinctive styles. By the 19th century, factory production of clock parts gradually made pendulum clocks affordable by middle-class families.
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| During the [[Industrial Revolution]], daily life was organized around the home pendulum clock. More accurate pendulum clocks, called ''regulators'', were installed in places of business and used to schedule work and set other clocks. The most accurate, known as ''astronomical regulators'', were used in observatories for astronomy, surveying, and [[celestial navigation]]. Beginning in the 19th century, astronomical regulators in naval observatories served as primary standards for national [[Time signal|time distribution services]].<ref>Milham 1945, p.83</ref> From 1909, US National Bureau of Standards (now [[NIST]]) based the US time standard on [[Riefler escapement|Riefler]] pendulum clocks, accurate to about 10 milliseconds per day. In 1929 it switched to the [[Shortt-Synchronome clock|Shortt-Synchronome free pendulum clock]] before phasing in [[Quartz clock|quartz]] standards in the 1930s.<ref>{{cite web
| |
| | date=April 30, 2002
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| | url=http://physics.nist.gov/GenInt/Time/revol.html
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| | title=A Revolution in Timekeeping
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| | publisher=Time and Frequency Services, NIST
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| | accessdate=2007-05-29 |archiveurl = http://web.archive.org/web/20070528005441/http://physics.nist.gov/GenInt/Time/revol.html <!-- Bot retrieved archive --> |archivedate = 2007-05-28}}</ref>
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| <ref>{{cite conference
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| |last=Sullivan
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| |first=D.B.
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| |title=Time and frequency measurement at NIST: The first 100 years
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| |year=2001
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| |booktitle=2001 IEEE Int'l Frequency Control Symp.
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| |publisher=National Institute of Standards and Technology
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| |url=http://tf.nist.gov/timefreq/general/pdf/1485.pdf|format=PDF}}</ref> With an error of around one second per year, the Shortt was the most accurate commercially-produced pendulum clock.<ref name="Jones">{{cite book
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| | last = Jones
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| | first = Tony
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| | authorlink =
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| | coauthors =
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| | title = Splitting the Second: The Story of Atomic Time
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| | publisher = CRC Press
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| | year = 2000
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| | location = US
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| | pages = 30
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| | url = http://books.google.com/?id=krZBQbnHTY0C&pg=PA30
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| | doi =
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| | id =
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| | isbn = 978-0-7503-0640-9}}</ref><ref>{{cite book
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| | last = Milham
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| | first = Willis I.
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| | authorlink =
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| | coauthors =
| |
| | title = Time and Timekeepers
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| | publisher = MacMillan
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| | year = 1945
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| | location = New York
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| | pages = 615
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| | url =
| |
| | doi =
| |
| | id =
| |
| | isbn = }}</ref><ref name="Marrison">{{cite journal
| |
| |last=Marrison
| |
| |first=Warren
| |
| |title=The Evolution of the Quartz Crystal Clock
| |
| |journal=Bell System Technical Journal
| |
| |year=1948
| |
| |volume=27
| |
| |pages=510–588
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| |url=http://www.ieee-uffc.org/main/history.asp?file=marrison}}</ref><ref>{{cite web
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| | last =
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| | first =
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| | authorlink =
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| | coauthors =
| |
| | title = The Reifler and Shortt clocks
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| | work =
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| | publisher = [http://www.clockvault.com/index.html JagAir Institute of Time and Technology]
| |
| | date =
| |
| | url = http://www.clockvault.com/heritage/index.htm
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| | doi =
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| | accessdate = 2009-12-29}}</ref><ref name="Betts">{{cite web
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| | last = Betts
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| | first = Jonathan
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| | authorlink =
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| | coauthors =
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| | title = Expert's Statement, Case 6 (2008-09) William Hamilton Shortt regulator
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| | work = Export licensing hearing, Reviewing Committee on the Export of Works of Art and Objects of Cultural Interest
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| | publisher = [http://www.mla.gov.uk/ UK Museums, Libraries, and Archives Council]
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| | date = May 22, 2008
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| | url = http://www.mla.gov.uk/what/cultural/export/reviewing_cttee/~/media/Files/word/2009/RCEWA/Cases%202008-09/Case%206%202008-09%20Regulator/internet%20experts%20statement%20shortt.ashx
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| | format = DOC
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| | doi =
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| | accessdate = 2009-12-29}}</ref>
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| | |
| Pendulum clocks remained the world standard for accurate timekeeping for 270 years, until the invention of the [[quartz clock]] in 1927, and were used as time standards through World War 2. The French Time Service used pendulum clocks as part of their ensemble of standard clocks until 1954 <ref>{{cite book
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| | last = Audoin
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| | first = Claude
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| | authorlink =
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| | coauthors = Bernard Guinot, Stephen Lyle
| |
| | title = The Measurement of Time: Time, Frequency, and the Atomic Clock
| |
| | publisher = Cambridge Univ. Press
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| | year = 2001
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| | location = UK
| |
| | pages = 83
| |
| | url = http://books.google.com/?id=LqdgUcm03A8C
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| | doi =
| |
| | id =
| |
| | isbn = 0-521-00397-0}}</ref> The home pendulum clock began to be replaced as domestic timekeeper during the 1930s and 1940s by the synchronous [[electric clock]], which kept more accurate time because it was synchronized to the oscillation of the [[electric power grid]]. The most accurate experimental pendulum clock to date (2007) may be the Littlemore clock, built by [[Edward Thomas Hall|Edward T. Hall]] in the 1990s <ref>{{cite web
| |
| |last=Hall
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| |first=E.T.
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| |title=The Littlemore Clock
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| |date=June 1996
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| |work=[http://www.iinet.com/~holmstro/hsn_entry.html NAWCC Chapter #161 Horological Science]
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| |publisher=Nat'l Assoc. of Watch and Clock Collectors
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| |pages=fig. 7b
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| |url=http://www.hsn161.com/HSN/hsn_article.php}}</ref>
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| (donated in 2003 to the [[National Watch and Clock Museum]], Columbia, USA).
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| '''
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| | |
| ==Mechanism==
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| [[File:Rivaz marine clock circa 1750.jpg|thumb|[[Pierre de Rivaz]] marine clock circa 1750.]]
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| All mechanical pendulum clocks have these five parts:<ref>Milham 1945, p.74, 197–212</ref>
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| * A power source; either a weight on a cord or chain that turns a pulley or sprocket, or a [[mainspring]]
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| * A [[gear train]] ([[wheel train (horology)|wheel train]]) that steps up the speed of the power so that the pendulum can use it
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| * An [[escapement]] that gives the pendulum precisely timed impulses to keep it swinging, and which releases the gear train wheels to move forward a fixed amount at each swing. This is the source of the "ticking" sound of an operating pendulum clock.
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| * The pendulum, a weight on a rod
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| * An indicator or dial that records how often the escapement has rotated and therefore how much time has passed, usually a traditional [[clock face]] with rotating hands.
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| Additional functions in clocks besides basic timekeeping are called [[complication (horology)|complication]]s. More elaborate pendulum clocks may include these complications:
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| * [[Striking clock|Striking train]] – strikes a chime on every hour, with the number of strikes equal to the number of the hour. More elaborate types, technically called ''chiming clocks'', strike on the quarter hours, and may play tunes, usually [[Westminster quarters]].
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| * Calendar dials – show the day, date, and sometimes month
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| * [[Moon phase]] dial – Shows the phase of the moon, usually with a painted picture of the moon on a rotating disk.
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| * [[Equation of time]] dial – this rare complication was used in early days to set the clock by the passage of the sun overhead at noon. It displays the difference between the time indicated by the clock and the time indicated by the position of the sun, which varies by as much as ±16 minutes during the year.
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| * [[Repeater (horology)|Repeater]] attachment – repeats the hour chimes when triggered by hand. This rare complication was used before artificial lighting to check what time it was at night.
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| In ''electromechanical pendulum clocks'' such as used in mechanical [[Master clock]]s the power source is replaced by an electrically powered [[solenoid]] that provides the impulses to the pendulum by [[magnetic force]], and the escapement is replaced by a [[switch]] or [[photodetector]] that senses when the pendulum is in the right position to receive the impulse. These should not be confused with more recent quartz pendulum clocks in which an electronic [[quartz clock]] module swings a pendulum. These are not true pendulum clocks because the timekeeping is controlled by a [[quartz crystal]] in the module, and the swinging pendulum is merely a decorative simulation.
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| ==Gravity-swing pendulum==
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| [[Image:Old Pendulum clock.jpg|thumb|Schoolhouse regulator style pendulum wall clock]]
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| The pendulum swings with a [[Frequency|period]] that varies with the square root of its effective length. For small swings the period ''T'' in seconds, the time for one complete cycle (two swings), is
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| :<math>T = 2 \pi \sqrt{\frac{L}{g}} \,</math>
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| where ''L'' is the length of the pendulum in meters and ''g'' is the local [[gravitational acceleration|acceleration of gravity]] in meters per second squared. All pendulum clocks have a means of adjusting the rate. This is usually an adjustment nut under the pendulum [[Bob (physics)|bob]] which moves the bob up or down on its rod. Moving the bob up reduces the length of the pendulum, reducing the pendulum's period so the clock gains time. In some pendulum clocks, fine adjustment is done with an auxiliary adjustment, which may be a small weight that is moved up or down the pendulum rod. In some master clocks and tower clocks, adjustment is accomplished by a small tray mounted on the rod where small weights are placed or removed to change the effective length, so the rate can be adjusted without stopping the clock.
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| The period of a pendulum increases slightly with the width (amplitude) of its swing. The ''rate'' of error increases with amplitude, so when limited to small swings of a few degrees the pendulum is nearly ''isochronous''; its period is independent of changes in amplitude. Therefore the swing of the pendulum in clocks is limited to 2° to 4°. | |
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| ===Thermal compensation===
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| One source of error in clocks is that the pendulum rod changes length slightly with changes in temperature. An increase in temperature causes the rod to expand, making the pendulum longer, so its period increases and the clock loses time. Wood expands less than metal, so many older quality clocks had wooden pendulum rods. To compensate for this expansion, early high-precision clocks used ''mercury pendulums'', invented by [[George Graham (clockmaker)|George Graham]] in 1721. These had a bob consisting of a container of the liquid metal [[Mercury (element)|mercury]]. An increase in temperature would cause the rod to expand, but the mercury in the container would expand also and its level would rise slightly in the container, keeping the center of gravity of the pendulum at the same height.
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| The most widely used temperature compensated pendulum was the [[gridiron pendulum]] invented by [[John Harrison]] in 1726. This consisted of a "grid" of parallel rods of high-thermal-expansion metal such as [[zinc]] or [[brass]] and low-thermal-expansion metal such as [[steel]] mounted in a frame. It was constructed so that the high expansion rods compensated for the length change of the low-expansion rods, achieving zero length change with temperature changes. This type of pendulum became so associated with quality that decorative "fake" gridirons are often seen on pendulum clocks, that have no actual temperature compensation function.
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| A few of the highest precision scientific clocks, constructed around 1900, had "high tech" pendulums of low-expansion materials such as the nickel steel alloy [[Invar]] and [[fused silica]].
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| ===Atmospheric drag===
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| The viscosity of the air through which the pendulum swings will vary with atmospheric pressure, humidity, and temperature. This drag also requires power that could otherwise be applied to extending the time between windings. Pendulums are sometimes polished and streamlined to reduce the effects of air drag (which is where most of the driving power goes) on the clock's accuracy. In the late 19th century and early 20th century, pendulums for clocks in astronomical observatories were often operated in a chamber that had been pumped to a low pressure to reduce drag and make the pendulum's operation even more accurate.
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| ===Leveling and 'beat'===
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| To keep time accurately, pendulum clocks must be absolutely level. If they are not, the pendulum swings more to one side than the other, upsetting the symmetrical operation of the escapement. This condition can often be heard audibly in the ticking sound of the clock. The ticks or 'beats' should be at precisely equally spaced intervals to give a sound of, "tick...tock...tick...tock"; if they are not, and have the sound "tick-tock...tick-tock..." the clock is ''out of beat'' and needs to be leveled. This problem can easily cause the clock to stop working, and is one of the most common reasons for service calls. A [[spirit level]] or [[watch timing machine]] can achieve a higher accuracy than relying on the sound of the beat; precision regulators often have a built in spirit level for the task. Older freestanding clocks often have feet with adjustable screws to level them, more recent ones have a leveling adjustment in the movement. Some modern pendulum clocks have 'auto-beat' or 'self-regulating beat adjustment' devices, and don't need this adjustment.
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| ===Local gravity===
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| Since the pendulum rate will increase with an increase in gravity, and local gravity varies with latitude and elevation on Earth, pendulum clocks must be readjusted to keep time after a move. For example, a pendulum clock moved from sea level to 4000 feet will lose 16 seconds per day.<ref>{{cite web
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| | last = Arnstein
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| | first = Walt
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| | authorlink =
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| | coauthors =
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| | title = The Gravity Pendulum and its Horological Quirks
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| | work = Community Articles
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| | publisher = Timezone.com website
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| | date =
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| | url = http://www.timezone.com/library/comarticles/comarticles0013
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| | format =
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| | doi =
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| | accessdate = 2011-04-01}}</ref> Even moving a clock to the top of a tall building will cause it to lose measurable time due to lower gravity.<ref>{{cite journal
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| | last = Gore
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| | first = Jeff
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| | authorlink =
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| | coauthors = Alexander van Oudenaarden
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| | title = The Yin and Yang of Nature
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| | journal = Nature
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| | volume = 457
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| | issue = 7227
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| | pages = 271
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| | publisher = MacMillan
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| | location =
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| | date = January 15, 2009
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| | url = http://web.mit.edu/biophysics/papers/NATURENV2009.pdf
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| | issn =
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| | doi = 10.1038/457271a
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| | id =
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| | accessdate = 2009-07-22
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| | pmid = 19148089|bibcode = 2009Natur.457..271G }}</ref>
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| | |
| ==Torsion pendulum==
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| {{main|Torsion pendulum clock}}
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| Also called torsion-spring pendulum, this is a wheel-like mass (most often four spheres on cross spokes) suspended from a vertical strip (ribbon) of spring steel, used as the regulating mechanism in [[torsion pendulum clock]]s. Rotation of the mass winds and unwinds the suspension spring, with the energy impulse applied to the top of the spring. As the period of a cycle is quite slow compared to the gravity swing pendulum, it is possible to make clocks that need to be wound only every 30 days, or even only once a year or more. A clock requiring only annual winding is sometimes called a "'''400-Day clock"''', "'''perpetual clock'''" or "'''anniversary clock'''", the latter sometimes given as a wedding memorialisation gift. Schatz and Kundo, both German firms, were once the main manufacturers of this type of clock<!-- ; it is no longer being produced -->. This type is independent of the local force of gravity but is more affected by temperature changes than an uncompensated gravity-swing pendulum.
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| [[File:Ansonia Clock Co.jpg|thumb|Clock Ansonia, model: C.1904, santiago.]]
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| ==Escapement==
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| {{main|escapement}}
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| | |
| The escapement drives the pendulum, usually from a [[gear]] train, and is the part that ticks. Most escapements have a locking state and a drive state. In the locking state, nothing moves. The motion of the pendulum switches the escapement to drive, and the escapement then pushes on the pendulum for some part of the pendulum's cycle. A notable but rare exception is Harrison's [[grasshopper escapement]]. In precision clocks, the escapement is often driven directly by a small weight or spring that is re-set at frequent intervals by an independent mechanism called a [[remontoire]]. This frees the escapement from the effects of variations in the gear train. In the late 19th century, electromechanical escapements were developed. In these, a mechanical switch or a [[phototube]] turned an [[electromagnet]] on for a brief section of the pendulum's swing. These were used on some of the most precise clocks known. They were usually employed with [[vacuum]] pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.
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| In the 20th century, [[W.H. Shortt]] invented a [[Escapement#Free pendulum clock|free pendulum clock]] / [[Shortt-Synchronome clock]]
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| with an accuracy of one-hundredth of a second per day. In this system, the timekeeping pendulum does no work and is kept swinging by a push from a weighted arm (gravity arm) that is lowered onto the pendulum by another (slave) clock just before it is needed. The gravity arm then pushes on the free pendulum, which releases it to drop out of engagement at a time that is set entirely by the free pendulum. Once the gravity arm is released, it trips a mechanism to reset itself ready for release by the slave clock. The whole cycle is kept synchronised by a small blade spring on the pendulum of the slave clock. The slave clock is set to run slightly slow, and the reset circuit for the gravity arm activates a pivoted arm that just engages with the tip of the blade spring. If the slave clock has lost too much time, its blade spring pushes against the arm and this accelerates the pendulum. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became the standard for use in observatories from the mid-1920s until superseded by quartz technology.
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| ==Time Indication==
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| [[File:Mantel clock by Julien Béliard.jpg|thumb|[[Mantel clock]] (around 1800) by Julien Béliard, [[Paris]], maître horloger recorded on the rue Saint-Benôit and rue Pavée in 1777, still active in 1817, or Julien-Antoine Béliard, maître horloger in 1786, recorded on the rue de Hurepoix, 1787-1806.]]
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| The indicating system is almost always the traditional [[Clock face|dial]] with moving hour and minute hands. Many clocks have a small third hand indicating seconds on a subsidiary dial. Pendulum clocks are usually designed to be set by opening the glass face cover and manually pushing the minute hand around the dial to the correct time. The minute hand is mounted on a slipping friction sleeve which allows it to be turned on its arbor. The hour hand is driven not from the [[wheel train (horology)|wheel train]] but from the minute hand's shaft through a small set of gears, so rotating the minute hand manually also sets the hour hand.
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| ==Styles==
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| Pendulum clocks were more than simply utilitarian timekeepers; they were [[status symbol]]s that expressed the wealth and culture of their owners. They evolved in a number of traditional styles, specific to different countries and times as well as their intended use. Case styles somewhat reflect the furniture styles popular during the period. Experts can often pinpoint when an [[antique]] clock was made within a few decades by subtle differences in their cases and faces. These are some of the different types of pendulum clocks:
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| {{div col|colwidth=22em}}
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| *[[Act of Parliament clock]]
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| *[[Banjo clock]]
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| *[[Bracket clock]]
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| *[[Cartel clock]]
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| *[[Longcase Clock#Comtoise clocks|Comtoise or Morbier clock]]
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| *Crystal regulator
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| *[[Cuckoo clock]]
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| *[[Longcase clock]] (commonly known as a grandfather clock)
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| *[[Lantern clock]]
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| *[[Mantel clock]]
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| *[[Master clock]]
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| *[[Ogee|Ogee clock]]
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| *Pillar clock
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| *[[Schoolhouse regulator]]
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| *[[Shortt-Synchronome clock]]
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| *Torsion pendulum clock (uses a torsion pendulum)
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| *[[Turret clock]]
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| *Vienna regulator
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| *Zaandam clock
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| {{div col end}}
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| ==See also==
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| *[[Steam clock]]
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| ==References==
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| {{reflist|33em}}
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| ==External links==
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| {{commons category|Pendulum clocks}}
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| *[http://inventors.about.com/library/weekly/aa072801a.htm The Invention of Clocks]
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| *[http://myreckonings.com/wordpress/2007/11/19/the-not-so-simple-pendulum/ The (Not So) Simple Pendulum]
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| * [http://www.kunstpedia.com/articles/46/1/The-earliest-DUTCH-and-FRENCH-Pendulum-clocks-1657-1662/Page1.html The earliest DUTCH and FRENCH * https://docs.google.com/file/d/0B9pkEBzqr_M5MGtmc2hQcUYwSXc/edit?usp=sharing Pendulum clocks, 1657-1662]
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| {{DEFAULTSORT:Pendulum Clock}}
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| [[Category:Clocks]]
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| [[Category:Horology]]
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| [[Category:Pendulums]]
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| [[Category:Dutch inventions]]
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| [[Category:Obsolete technologies]]
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