FOUCAULT PENDULUM

relatively large mass suspended from a long wire mounted so that its perpendicular plane of swing is not confined to a particular direction and, in fact, rotates in relation to the Earth's surface. Jean-Bernard-Lon Foucault assembled (1851) in Paris the first pendulums of this type, one of which consisted of a 28-kilogram (62-pound) iron ball suspended from the dome of the Panthon by a steel wire 67 m (220 feet) long and kept in motion by a mechanism. The rotation of the plane of swing of Foucault's pendulums was the first laboratory demonstration of the Earth's spin on its axis. While any Foucault pendulum swings back and forth in a plane, the Earth rotates beneath it, so that relative motion exists between them. At the North Pole, latitude 90 north, the relative motion as viewed from the plane of the pendulum's suspension is a counterclockwise rotation of the Earth once every 24 hours; whereas the plane of the pendulum as viewed from the Earth looking upward rotates in a clockwise direction once a day. A Foucault pendulum always rotates clockwise in the Northern Hemisphere with a rate that becomes slower as the Equator is approached. Foucault's original pendulums at Paris rotated clockwise at a rate of more than 11 per hour, or with a period of about 32 hours per complete rotation. The rate of rotation depends on the latitude. At the Equator, 0 latitude, a Foucault pendulum does not rotate. In the Southern Hemisphere, rotation is counterclockwise. The rate of rotation of a Foucault pendulum can be stated mathematically as equal to the rate of rotation of the Earth times the sine of the number of degrees of latitude. Because the Earth rotates once a day, or 360 every 24 hours, its rate of rotation may be expressed as 15 per hour, which corresponds to the rate of rotation of a Foucault pendulum at the North or South Pole. At latitude 30 north, for example, at Cairo or New Orleans, a Foucault pendulum would rotate at the rate of 7.5 per hour, for the sine of 30 is equal to one-half. The rate of rotation of a Foucault pendulum at any given point is, in fact, numerically equal to the component of the Earth's rate of rotation perpendicular to the Earth's surface at that point.