Foucault pendulum: see pendulum pendulum, a mass, called a bob, suspended from a fixed point so that it can swing in an arc determined by its momentum and the force of gravity. The length of a pendulum is the distance from the point of suspension to the center of gravity of the bob (see center of
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Foucault pendulum

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.

Foucault pendulum

A pendulum or swinging weight, supported by a long wire, by which J. B. L. Foucault demonstrated in 1851 the rotation of Earth on its axis. Foucault used a 62-lb (28-kg) iron ball suspended on about a 200-ft (60-m) wire in the Pantheon in Paris. The upper support of the wire restrains the wire only in the vertical direction. The bob is set swinging along a meridian in pure translation (no lateral or circular motion). In the Northern Hemisphere the plane of swing appears to turn clockwise; in the Southern Hemisphere it appears to turn counterclockwise, the rate being 15 degrees times the sine of the local latitude per sidereal hour. Thus, at the Equator the plane of swing is carried around by Earth and the pendulum shows no apparent rotation; at either pole the plane of swing remains fixed in space while Earth completes one rotation each sidereal day. See Pendulum