Editing Pendulum (section)

=== 1930: Decline in use ===
Around 1900 low-[[thermal expansion|thermal-expansion]] materials began to be used for pendulum rods in the highest precision clocks and other instruments, first [[invar]], a nickel steel alloy, and later [[fused quartz]], which made temperature compensation trivial.<ref name="BritannicaP540">{{cite EB1911|wstitle= Clock |volume= 06 |last= Penderel-Brodhurst |first= James George Joseph |author-link= James George Joseph Penderel-Brodhurst | pages = 536&ndash;553; see pages 540 and 541 |quote= }}</ref>  Precision pendulums were housed in low pressure tanks, which kept the air pressure constant to prevent changes in the period due to changes in [[buoyancy]] of the pendulum due to changing [[atmospheric pressure]].<ref name="BritannicaP540" />  The best pendulum clocks achieved accuracy of around a second per year.<ref name="Jones2000">{{cite book
  | last = Jones
  | first = Tony
  | title = Splitting the Second: The Story of Atomic Time
  | publisher = CRC Press
  | year = 2000
  | page = 30
  | url = https://books.google.com/books?id=krZBQbnHTY0C&pg=PA30
  | isbn = 978-0-7503-0640-9}}</ref><ref>{{cite book
  | last = Kaler
  | first = James B.
  | title =  Ever-changing Sky: A Guide to the Celestial Sphere
  | publisher = Cambridge Univ. Press
  | year = 2002
  | location = UK
  | page = 183
  | url = https://books.google.com/books?id=KYLSMsduNqcC&pg=PA183
  | isbn = 978-0-521-49918-7}}</ref>

The timekeeping accuracy of the pendulum was exceeded by the [[quartz]] [[crystal oscillator]], invented in 1921, and [[quartz clock]]s, invented in 1927, replaced pendulum clocks as the world's best timekeepers.<ref name="Marrison" />  Pendulum clocks were used as time standards until World War 2, although the French Time Service continued using them in their official time standard ensemble until 1954.<ref>{{cite book
  | last = Audoin
  | first = Claude
  |author2=Bernard Guinot |author3=Stephen Lyle
  | title = The Measurement of Time: Time, Frequency, and the Atomic Clock
  | publisher = Cambridge Univ. Press
  | year = 2001
  | location = UK
  | page = 83
  | url = https://books.google.com/books?id=LqdgUcm03A8C
  | isbn = 978-0-521-00397-1}}</ref>  Pendulum [[gravimeter]]s were superseded by "free fall" gravimeters in the 1950s,<ref name="Torge">{{cite book
  | last = Torge
  | first = Wolfgang
  | title = Geodesy: An Introduction
  | publisher = Walter de Gruyter
  | year = 2001
  | page = 177
  | url = https://books.google.com/books?id=pFO6VB_czRYC&pg=PA177
  | isbn = 978-3-11-017072-6}}</ref> but pendulum instruments continued to be used into the 1970s.