Foucault demonstrating an early example of his pendulum in the Pantheon, Paris.
The Foucault pendulum is a simple pendulum consisting of a heavy metal bob on a long, fine wire attached to a support, that shows the rotation of the Earth. As the Earth turns on its axis, the plane of swing of the pendulum, as seen from the Earth's surface, rotates because of the bob's inertial resistance (see inertia) to change of absolute direction, until it has returned to its original orientation. Its period of rotation depends on its latitude.
The Foucault pendulum was first demonstrated by Léon Foucault in 1851 at the Paris World's Fair.