﻿

# Foucault pendulum

a pendulum that demonstrates the rotation of the earth by exhibiting an apparent change in its plane of oscillation.
[1850-55; named after J.B.L. FOUCAULT]

* * *

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.

* * *

relatively large mass suspended from a long line 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. In 1851 the French physicist Jean-Bernard-Léon Foucault (Foucault, Jean) assembled in Paris the first pendulums of this type, one of which consisted of a 28-kg (62-pound) iron ball suspended from inside the dome of the Panthéon by a steel wire 67 metres (220 feet) long and set in motion by drawing the ball to one side and carefully releasing it to start it swinging in a plane. 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 a 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° N, the relative motion as viewed from above in the plane of the pendulum's suspension is a counterclockwise rotation of the Earth once approximately every 24 hours (more precisely, once every 23 hours 56 minutes 4 seconds, the length of a sidereal day). Correspondingly, the plane of the pendulum as viewed from above appears to rotate 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 pendulum's location approaches the Equator. 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 sidereal day, or 360° approximately 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° N—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.

* * *

Universalium. 2010.

### Look at other dictionaries:

• Foucault pendulum — n. a pendulum consisting of a heavy weight on the end of a long wire hanging from a fixed point, of the kind invented by Jean Foucault to show that the earth is rotating: the pattern of the pendulum s swinging appears to be rotating to an… …   English World dictionary

• Foucault pendulum — The Foucault pendulum (pronEng|fuːˈkoʊ foo KOH ), or Foucault s pendulum, named after the French physicist Léon Foucault, was conceived as an experiment to demonstrate the rotation of the Earth.The experimentThe experimental apparatus consists of …   Wikipedia

• Foucault pendulum — Fuko švytuoklė statusas T sritis fizika atitikmenys: angl. Foucault pendulum vok. Foucaultsches Pendel, n rus. маятник Фуко, m pranc. pendule de Foucault, m …   Fizikos terminų žodynas

• Foucault pendulum — noun pendulum with a long wire; can swing in any direction; the change in the swing plane demonstrates the earth s rotation • Hypernyms: ↑pendulum * * * noun Usage: usually capitalized F Etymology: after J.B.L. Foucault : a freely swinging… …   Useful english dictionary

• Foucault pendulum — noun Etymology: J.B.L. Foucault Date: 1931 a freely swinging pendulum that consists of a heavy weight hung by a long wire and that swings in a constant direction which appears to change showing that the earth rotates …   New Collegiate Dictionary

• Foucault pendulum —  For the device (pronounced foo ko), but Foucault’s Pendulum (1988) for the novel by Umberto Eco …   Bryson’s dictionary for writers and editors

• Foucault pendulum — /fukoʊ ˈpɛndʒələm/ (say foohkoh penjuhluhm) noun a long simple pendulum which demonstrates the rotation of the earth by changing its plane of oscillation during the course of a day. {demonstrated by Jean Bernard Léon Foucault, 1819–68, French… …   Australian English dictionary

• Foucault pendulum — noun Foucaults pendulum …   Wiktionary

• Foucault Pendulum vector diagrams — Several vector diagrams are often used to demonstrate the physics underlying the Foucault pendulum.Diagrams are provided to illustrate a pendulum located at the North Pole, equator, and 45 degrees N to show how the rotation of Earth in relation… …   Wikipedia

• Foucault pendulum vector diagrams — Several vector diagrams are often used to demonstrate the physics underlying the Foucault pendulum. Diagrams are provided to illustrate a pendulum located at the North Pole, equator, and 45 degrees N to show how the rotation of Earth in relation… …   Wikipedia

### Share the article and excerpts

Do a right-click on the link above