Editing Pendulum (section)

=== 1721: Temperature compensated pendulums ===
[[File:Foucault pendulum animated.gif|thumb|right|The [[Foucault pendulum]] in 1851 was the first demonstration of the Earth's rotation that did not involve celestial observations, and it created a "pendulum mania". In this animation the rate of precession is greatly exaggerated.]]

During the 18th and 19th century, the [[pendulum clock]]'s role as the most accurate timekeeper motivated much practical research into improving pendulums. It was found that a major source of error was that the pendulum rod expanded and contracted with changes in ambient temperature, changing the period of swing.<ref name="Milham1945" /><ref name="Beckett1874">{{cite book
|last=Beckett
|first=Edmund
|author-link= Edmund Beckett, 1st Baron Grimthorpe
|year=1874
|title=A Rudimentary Treatise on Clocks and Watches and Bells, 6th Ed
|publisher=Lockwood & Co.
|location=London
|url=https://books.google.com/books?id=OvQ3AAAAMAAJ&pg=PA50
|page=50  }}</ref>  This was solved with the invention of temperature compensated pendulums, the mercury pendulum in 1721<ref name="Graham1726">{{cite journal
  | last = Graham
  | first = George
  | title = A contrivance to avoid irregularities in a clock's motion occasion'd by the action of heat and cold upon the rod of the pendulum
  | journal = Philosophical Transactions of the Royal Society
  | volume = 34
  | issue = 392–398
  | pages = 40–44
  | year = 1726
  | url =https://zenodo.org/record/1432208
  | doi = 10.1098/rstl.1726.0006
  | s2cid = 186210095
  | doi-access = free
  }} cited in {{cite book
  | last = Day
  | first = Lance
  |author2=Ian McNeil
  | title = Biographical Dictionary of the History of Technology
  | publisher = Taylor & Francis
  | year = 1996
  | page = 300
  | url = https://books.google.com/books?id=UuigWMLVriMC&pg=PA300
  | isbn =  978-0-415-06042-4}}</ref> and the [[gridiron pendulum]] in 1726, reducing errors in precision pendulum clocks to a few seconds per week.<ref name="Andrewes1994" />

The accuracy of gravity measurements made with pendulums was limited by the difficulty of finding the location of their [[Center of percussion|center of oscillation]]. Huygens had discovered in 1673 that a pendulum has the same period when hung from its center of oscillation as when hung from its pivot,<ref name="HuygensReciprocity" /> and the distance between the two points was equal to the length of a simple gravity pendulum of the same period.<ref name="HuygensCompound" />  In 1818 British Captain [[Henry Kater]] invented the reversible [[Kater's pendulum]]<ref name="Kater1818">{{cite journal
  | last = Kater
  | first = Henry
  | title = An account of experiments for determining the length of the pendulum vibrating seconds in the latitude of London
  | journal = Phil. Trans. R. Soc.
  | volume = 104
  | issue = 33
  | page = 109
  | year = 1818
  | url = https://books.google.com/books?id=uaQOAAAAIAAJ&q=%22Henry+Kater%22+kater+pendulum&pg=PA83
  | access-date = 2008-11-25}}</ref> which used this principle, making possible very accurate measurements of gravity. For the next century the reversible pendulum was the standard method of measuring absolute gravitational acceleration.