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What is the Foucault Pendulum?

By D.R. Satori
Updated May 23, 2024
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A Foucault pendulum, like other pendulums, consists of a weight hanging from a rope or wire. The weight is lifted, then let go, and the pendulum swings back and forth in the vertical plane, until friction with the attachment point above, or the air surrounding it, slows it down. Then the pendulum, like a swing, needs another push to get it going again. Sometime before the mid 19th century, physicist Leon Foucault realized that if he could extend the time a pendulum swings before it stops and remove the friction from its attachment above, the Earth could be seen to rotate out from under it, thus demonstrating that it was the Earth rotating under the sky and not the other way around.

Foucault knew that the longer the wire attached to the weight, the slower the swing of his pendulum would be. He could attach the rope above to an almost frictionless pin, and if he used a very large weight, the pendulum would swing so slowly that it would appear to change direction, as the Earth moved out from under it. The drawback was that because the Foucault pendulum was the largest pendulum ever created, Foucault needed a building with a very high ceiling. The Pantheon, a giant mausoleum with a columned entrance and high domed ceiling was offered, and the Foucault pendulum had a home.

The Foucault pendulum not only proves that the Earth rotates about its axis, but like a gyroscope, a freely rotating machine that does not change direction unless pushed, it illustrates Newton’s first law of motion. The reason the Earth can move out from under the pendulum is that the pendulum is set in motion and will not change its motion to match that of the Earth, unless some additional force acts on it to do so. There was a problem, though, with the demonstration. The Foucault pendulum at the Pantheon in Paris, France took over thirty hours to reach its original position, more than the time required for the Earth to complete one rotation.

A simple equation tells researchers the latitude required for the Foucault pendulum to be most effective, and the equation predicts that, at the equator, the Earth will not rotate out from under the pendulum. If Leon Foucault had done his experiment at the equator, he would have seen no change in direction of the pendulum, only a back and forth movement. The North and South Poles are also ideal places for the experiment. At the North Pole, as though suspended from the North Star, the Foucault pendulum would allow the Earth to rotate out from under it in exactly 24 hours. Videos are available online to illustrate the gradations of motion of the Foucault pendulum between equator and poles.

Today, there are thousands of Foucault pendulums housed mostly in universities and scientific institutions all over the world. These pendulums will not be in perpetual motion, since through loss of energy, the weight will rise to lower and lower levels until it points downward to the center of the Earth and stops. To start the pendulum, originally, a rope was used to lift the weight to a maximum height. The rope was then set on fire, so that when it burned through, no additional forces affected the pendulum’s movement on startup. Modern pendulums usually employ magnetic devices to start and keep the pendulum in motion.

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