Fig. 8
The essential features of our Foucault pendulum, which is based on Crane's design, are shown in Fig. 8. Of particular note is the Charron ring used to limit the elipticity and the pair of magnets A and B with magnet B adjustable to alter the force law and reduce the precession resulting from any residual elipticity. Finally there is the magnetic drive coil which is driven in phase with the pendulum and creates a push-pull force on magnet A to keep the pendulum going against friction. The drive current is controlled by electronics under the base which are in turn triggered by an induction current in the sense coil every time the pendulum magnet A sweeps by. An LED under the base support flashes on whenever the drive is in its push mode.

   The various features are:
     A Permanent magnet fixed in the bob stem
     B Adjustable height permanent magnet
     C Charron ring: mass = 160 gm
     D Drive coil
     E Sense coil
     F Pointer: mass = 1/3 gm
     G Pendulum bob: mass = 4.5 kgm
     H Wire: length to bob center of mass = 83 cm
     I Pin vice wire holder
     J Safety hanger for protection against wire breaks

The Charron ring limits the pendulum swing angle to = 3.9º.

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Pendulum