Magnetic monitoring of a small Foucault pendulum.

A three-axis magnetometer is used to measure the precession angle of a small Foucault pendulum of 65.4 cm length and period 1.623 s. The swinging brass bob (4 kg) contains a small neodymium magnet to detect and sustain its motion. A microcontroller is used to control electronics that drive the pendulum while sampling the time-dependent magnetic field from the swinging bob. These data are used to calculate the precession angle for each pendulum period. Long-term studies demonstrated a precession rate of 10.31°/h which is within 0.5% of that expected for the latitude of the experiment. Considerable short-term variation in the precession rate is observed (1.5°/h) which is correlated with the structure of the pendulum mechanics. The angle measurement noise is found to be 0.059° which enables a clear detection of the earth's rotation with only 26.7 s of observation.