For the novel Foucault's Pendulum by Umberto Eco, See Foucault's Pendulum (book).

Foucault's Pendulum in the Panthéon, Paris

A Foucault pendulum, or Foucault's pendulum, named after the French physicist Léon Foucault, was conceived as an experiment to demonstrate the rotation of the Earth; its action is a result of the Coriolis force. It is a tall pendulum free to oscillate in any vertical plane and capable of running for many hours. The first Foucault pendulum exhibited to the public was in 1851, suspended by a 67-metre wire from the dome of the Panthéon in Paris.

At almost any location on Earth — except the equator — it can be observed that the plane within which the pendulum swings slowly rotates. At either the North Pole or South Pole, the plane of oscillation of a pendulum rotates once per sidereal day (in essence, the pendulum swing remains in the same plane while the Earth rotates underneath it, as predicted by Newton's first law of motion). At other latitudes, the plane of oscillation of a pendulum rotates with an angular speed proportional to the sine of its latitude; thus one at 45° rotates once every 1.4 days and one at 30° every 2 days.

Many people found the sine factor difficult to understand, which prompted Foucault to conceive the gyroscope in 1852. The gyroscope's spinning rotor tracks the stars directly. Its axis of rotation turns once per day whatever the latitude, unaffected by any sine factor.

A Foucault pendulum is tricky to set up because imprecise construction can cause additional veering which masks the terrestrial effect. Air resistance damps the oscillation, so Foucault pendulums in museums usually incorporate an electromagnetic or other drive to keep the bob swinging.

External links

• "About Foucault pendulums". California Academy of Sciences, 2003.
• "South Pole Foucault Pendulum". Winter, 2001.
• Weisstein, Eric W., "Foucault's pendulum".
• Wolfe, Joe, "A derivation of the precession of the Foucault pendulum".