The Speedometer

If a metal disc is rotated so that it cuts the magnetic field produced by a magnet, eddy currents are induced in the disc. Lenz's law tells us that the direction of the induced current is such as to oppose the change that caused it, so that the disk will experience a force in the opposite direction. The same principle can be used in the speedometers used in cars.

The disk above is actually a cup surrounding the magnet. The magnet is rotated by the input shaft at a speed which increases with the speed of the car. The reaction produced by the eddy currents sets up couple on the cap which rotates it – and the speed indicator – until it is balanced by the force exerted by a spring.