A speedometer is an instrument which measures the speed at which the car is travelling and usually also embodies a mileage recording mechanism.

The central feature of the device is a permanent magnet. Each magnet is surrounded by a magnetic field, which can be conceived as consisting of lines of force. These can be ‘made visible’ by strewing iron filings on a sheet of cardboard under which the magnet is held; the filings will arrange themselves along the lines of force and thus reveal the pattern of these lines. When a magnet is rotated, its field will rotate with it.

The magnet in the speedometer begins to rotate as soon as the vehicle is set in motion; it is driven through a small gear unit by the speedometer shaft, which is connected to the propeller shaft or the front axle. The higher the speed of the vehicle is, the higher is the speed of rotation of the magnet. The magnet rotates concentrically in an aluminium ring, in which the rotating magnetic field induces eddy currents which in turn produce a magnetic field of their own.

The interaction of this magnetic field with that of the rotating permanent magnet exercises a torque (twisting moment) on the aluminium ring. This torque tries to rotate the ring along with the magnet. The faster the magnet rotates, the higher is the torque.

The ring is not free to rotate, however: it can merely swing a certain distance - depending on the magnitude of the torque - and is then restrained by the counteracting force of a spiral spring. Attached to the ring is a pointer which indicates the speed of the vehicle on a suitably graduated scale. In many types of speedometer the ring has an extension in the form of a drum to which the scale is attached.

When the speed increases, this drum rotates, and the speed indication is shown in a narrow gap in a panel mounted in front of the scale.

The speedometer generally also comprises a mileage recorder. This device is driven through a small worm gear, which is mounted on the speedometer shaft, by the same shaft that drives the rotating magnet. The motion of the speedometer shaft, greatly reduced by another small gear unit, is transmitted to the mileage recording mechanism.

The latter therefore merely counts the number of revolutions. However, since a certain number of revolutions, of the front wheel or of the propeller shaft correspond to a certain distance travelled by the vehicle, the revolution counting mechanism can be made to give a direct reading of the mileage covered.

When the disc which counts the units (i.e., miles) has performed one whole revolution, the counting disc for the tens is rotated one place (by means of a small driving catch).

When the ‘tens’ disc has performed one revolution, it similarly rotates the ‘hundreds’ disc one place. This operation is repeated up to the ‘ten thousands’ disc.