A pendulum with precession, relative to an observer fixed to the earth, produced by the effect of Coriolis acceleration. If set swinging at the North Pole in a given plane in space, its linear momentum perpendicular to the plane is zero, and it will continue to swing in this invariable plane while the earth rotates beneath it with a period of one day. At a latitude L, the frequency of precession is 1/T sin L where T is one day.
The 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 effect. It is a tall pendulum free to oscillate in any vertical plane and ideally should include some sort of motor so that it can run continuously rather than have its motion damped by friction. The first Foucault pendulum exhibited to the public was in February 1851 in the Meridian Room of the Paris Observatory, although Vincenzo Viviani had already experimented with a similar device in 1661, .