Editing Kater's pendulum (section)

===Kater's solution===
However, in ''Horologium Oscillatorium'', Huygens had also proved that the pivot point and the center of oscillation were interchangeable. That is, if any pendulum is suspended upside down from its center of oscillation, it has the same period of swing, and the new center of oscillation is the old pivot point. The distance between these two conjugate points was equal to the length of a simple pendulum with the same period.

As part of a committee appointed by the [[Royal Society]] in 1816 to reform British measures, Kater had been contracted by the House of Commons to determine accurately the length of the seconds pendulum in London.<ref>{{cite book
  | last = Zupko
  | first = Ronald Edward
  | author-link = Ronald Edward Zupko
  | title = Revolution in Measurement: Western European Weights and Measures since the Age of Science
  | publisher = Diane Publishing
  | year = 1990
  | location = New York
  | pages = 107–110
  | url = https://books.google.com/books?id=twUNAAAAIAAJ&pg=PA110
  | isbn = 0-87169-186-8}}</ref> He realized Huygens' principle could be used to find the center of oscillation, and so the length ''L'', of a rigid (compound) pendulum. If a pendulum were hung upside down from a second pivot point that could be adjusted up and down on the pendulum's rod, and the second pivot were adjusted until the pendulum had the same period as it did when swinging right side up from the first pivot, the second pivot would be at the center of oscillation, and the distance between the two pivot points would be ''L''.
 
Kater was not the first to have this idea.<ref>[https://books.google.com/books?id=A1IqAAAAMAAJ&pg=RA2-PA315 Lenzen & Multauf 1964], p. 315</ref><ref>[https://books.google.com/books?id=TL4KAAAAIAAJ&pg=PA12# Poynting & Thompson 1907], p. 12</ref> French mathematician [[Gaspard de Prony]] first proposed a reversible pendulum in 1800, but his work was not published until 1889. In 1811 [[Bohnenberger|Friedrich Bohnenberger]] again discovered it, but Kater independently invented it and was first to put it in practice.
 
[[File:Kater pendulum.png|thumb|400px|Drawing of Kater's pendulum <br>''<span style="color:red;">(a)</span>'' opposing knife edge pivots from which pendulum is suspended <br>''<span style="color:red;">(b)</span>'' fine adjustment weight moved by adjusting screw <br>''<span style="color:red;">(c)</span>'' coarse adjustment weight clamped to rod by setscrew<br>''<span style="color:red;">(d)</span>'' bob <br>''<span style="color:red;">(e)</span>'' pointers for reading]]