page_title: Pendulum
section_title: 1721: Temperature compensated pendulums
hierarchical_section_title: Pendulum / History / 1721: Temperature compensated pendulums
caption_attribution_description: العربية: صورة متحركة لنواس فوكو توضح دوران الأرض حول محورها. Deutsch: Animation des Foucaultschen Pendels (Zeigt die Drehrichtung auf der südlichen Erdhalbkugel) English: Animation of a Foucault pendulum (showing the sense of rotation on the southern hemisphere) Français&#160;: Représentation animée d'un Pendule de Foucault, montrant le sens de rotation dans l'hémisphère sud. Suomi: Foucault'n heilurin pyörimisliike eteläisellä pallonpuoliskolla Español: Representación animada del Péndulo de Foucault, mostrando el sentido de rotación en el hemisferio sur. Bahasa Indonesia: Animasi bandul Foucault (menampilkan jejak rotasi di bumi bagian selatan) 日本語: フーコーの振り子の動画、南半球での動作を示す。背景の肖像はフーコー。 Nederlands: Animatie van de werking van de Slinger van Foucault, waarbij de afbeelding de draairichting van de aarde op het zuidelijk halfrond weergeeft. Українська: Анімація руху маятника Фуко, що показує добове обертання Землі у південній півкулі. Русский: Маятник Фуко — демонстрация суточного вращения Земли. Polski: Animacja wahadła Focaulta na południowej półkuli Türkçe: Foucault sarkacı animasyonu. Português: Representação animada do Pêndulo de Foucault, mostrando o sentido de rotação no hemisfério sul.
context_page_description: A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force acting on the pendulum's mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one complete cycle, a left swing and a right swing, is called the period. The period depends on the length of the pendulum and also to a slight degree on the amplitude, the width of the pendulum's swing.
  From the first scientific investigations of the pendulum around 1602 by Galileo Galilei, the regular motion of pendulums was used for timekeeping, and was the world's most accurate timekeeping technology until the 1930s. The pendulum clock invented by Christian Huygens in 1658 became the world's standard timekeeper, used in homes and offices for 270 years, and achieved accuracy of about one second per year before it was superseded as a time standard by the quartz clock in the 1930s.
context_section_description: 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.  This was solved with the invention of temperature compensated pendulums, the mercury pendulum in 1721 and the gridiron pendulum in 1726, reducing errors in precision pendulum clocks to a few seconds per week.
  The accuracy of gravity measurements made with pendulums was limited by the difficulty of finding the location of their 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, and the distance between the two points was equal to the length of a simple gravity pendulum of the same period.  In 1818 British Captain Henry Kater invented the reversible Kater's pendulum 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.
caption: 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.