Eddy-current speedometers have been used in cars, trucks and motorcycles since the early 20th century. In motorcycles, a flexible cable runs up from the wheel to the speedometer. At the speedometer, it is attached to a magnet. A small cup called the speedometer cup surrounds the magnet. The cup is attached to the speedometer wheel, which is attached to a spring.
When the wheel spins, it makes the cable spin, which makes the magnet spin. As the magnet spins, it creates rotating electronic currents called eddy currents in the speedometer cup. As the eddy currents move, they create a magnetic field in the cup.
How it Works
The magnetic field in the cup is attracted to the field in the magnet. As the magnet spins, it pulls on the cup. The faster the magnet spins, the greater the pull. The spring attached to the cup pulls back. When the cup is resting, the spring holds the speedometer in place at zero. As the wheel makes the magnet spin faster, it turns the dial further and further, overcoming the resistance of the spring. As the car motorcycle slows down, the spring pulls the needle back toward zero.