Mechanics > Relative Motion DCS# 1E20.10


 foucault pendulum

foucault pendulum frame with charron ring, pendulum, and electromagnet
next to 101-13
circuit box
variac 202-08-H
The rotation of the plane of a free-swinging pendulum shows that the surface of the rotating earth is not fixed in an inertial reference frame.  At Oberlin's latitude of 41.3 N, the plane of the swing precesses 9.9 per hour.

The electromagnet under the pendulum bob is switched on twice each cycle (when the pendulum wire contacts the charron ring) to replace the energy lost to friction.

This can be used in the display case or as a classroom demonstration.

Level the frame with the leveling feet.  Adjust the wire length so the bob is within a few millimeters of the magnet.  Adjust the variac voltage so that the amplitude of the swing remains constant.  The pendulum bob must be released without any tangential velocity.

The display case version includes a laser and photogate for tracking the motion over time:
laser pointer with power supply adapter 101-02-C1
DC power supply 202-08-D
photogate with external gate and connector
LabPro 107-02-B3
pc 113A

The laser is aimed along a diameter of the scale so that the pendulum crosses the beam twice each cycle.  The time the beam is blocked varies with the angle of the pendulum's plane of oscillation.  The photogate raw voltage is read and a running average plotted against time for 48 hours, then the graph restarts.


Wallace A. Hilton, "The Foucault Pendulum: A Corridor Demonstration," Am. J. Phys. vol 46, 436-438, (1978).

Haym Kruglak, et al.  "A Short Foucault Pendulum for a Hallway Exhibit," Am. J. Phys. vol. 46, 438-440, (1978).