Large pendulum that is free to swing in any direction.
As it swings back and forth, the earth rotates beneath it, so its perpendicular plane of swing rotates in relation to the earth's surface. Devised by J.-B.-L. Foucault in 1851, it provided the first laboratory demonstration that the earth spins on its axis. A Foucault pendulum always rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (a consequence of the Coriolis force ). The rate of rotation depends on the latitude, becoming slower as the pendulum is placed closer to the equator; at the equator, a Foucault pendulum does not rotate.