Purpose:Demonstrate operating principle of some speedometers.
Description:Two conductive discs are placed parallel and coaxial to each other, one mounted to a manual roator, the other suspended from a wire. The lower disc, on which a magnet is mounted, rotates when the handle is cranked. Induction causes the upper disc to rotate through an angle which is dependent on the torsional constant of the wire support and the rotational speed of the lower disc. The upper disc is damped by a magnet held in place on a tuning fork. The lower photographs above show details of the assembly.
This idea is used in some auto speedometers, where the wheels are attached to the lower disc and the speedometer indicator is attached to the upper disc.