As the magnet spins, it sets up a rotating magnetic field, creating forces that act on the speedcup. These forces cause electrical current to flow in the cup in small rotating eddies, Battery for Nihon Kohden ECG-6951E known as eddy currents. In some applications, eddy currents represent lost power and are therefore undesirable. But in the case of a speedometer, the eddy currents create a drag torque that does work on the speedcup. The cup and its attached needle turn in the same direction that the magnetic field is turning but only as far as the hairspring will allow it. The needle on the speedcup comes to a rest where the opposing force of the hairspring balances the force created by the revolving magnet.
What if the car increases or decreases its speed? If the car travels faster, the permanent magnet inside the speedcup will rotate faster, which creates a stronger magnetic field, larger eddy currents and a greater deflection of the speedometer needle. If the car slows down, the magnet inside the cup rotates more slowly, which reduces the strength of the magnetic field, resulting in smaller eddy currents and less deflection of the needle. When a car is stopped, the hairspring holds the needle at zero.
An electronic speedometer receives its data from a vehicle speed sensor (VSS), not a drive cable. The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a toothed metal disk and a stationary detector that covers a magnetic coil. As the teeth move past the coil, they "interrupt" the magnetic field, creating a series of pulses that are sent to a computer. For each 40,000 pulses from the VSS, the trip and total odometers increase by one mile. Speed is also determined from the input pulse frequency. Circuit electronics in the car are designed to display the speed either on a digital screen or on a typical analog system with a needle and dial.
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