After two earlier experiments, Foucault suspended a ball of 61 pounds on a steel wire 220 feet long from the dome of the Pantheon in Paris. Protruding from the ball was a pin adjusted to draw a mark through a rink of wet sand beneath the ball. Drawing the ball to one side, Foucault then released it.
With each swing it made a mark in the sand and appeared to rotate in a clockwise direction, thus demonstrating, as he predicted, that the pendulum would revolve about 270 degrees in 24 hours and show the rotation of the earth.
The same experiment in Indianapolis requires about 40 hours for a complete circle and approximately 24 hours for 240 degrees. This is due to the different latitudes and the length of the wire. (The longer the wire, the slower the bob swings and the less hindered by friction with the air.)
The time varies at other locations on the earth, also. At the North Pole, it would turn a full 360–degree circle in 24 hours, while at the equator it would not turn at all.
The pendulum seems to change its path during the day, but actually the floor beneath it changes by being twisted or moved counter–clockwise by the daily rotation of the earth.
Since air resistance would normally stop the pendulum after a few hours, an iron collar is installed on the wire near the top of the dome where the pendulum hangs. Surrounded by an electromagnet, the collar is attracted to the magnet as the bob swings out. As the iron collar touches the magnet, the magnet shuts off allowing the wire to swing back. This keeps the pendulum going but does not alter the path of the pendulum’s swing.
Because of slight uncontrolled forces, such as vibrations from the street, the bob sometimes swings in a narrow elliptical path one or two inches wide.