Speedometer (magnet-turning)

Function

The rotary magnet speedometer should display the speed, by transferring the rotary movement of a flexible shaft onto the permanent display of a speedometer needle. Thereby, the deviation of the needle should rise against a spring according to the revs of the speedo cable.

How it works

The flexible speedo cable is connected with a small bar magnet. It can be recognised in the drawing on top by its colours red for North Pole and green for South Pole. It rotates in a drum manufactured from aluminium, which is able to rotate, but not connected to the speedo cable. According to the rule of Lenz (1834) the rotary magnet in the aluminium drum induces eddy currents which become stronger with rising revs. The bar magnet tries to transfer its rotary movement onto the aluminium drum, the more, the faster it turns. Thus the drum is turned against a spring at the end of the shaft with rising revs of the speedo cable. This increases the deviation of the speedometer needle which is connected to the aluminium drum. The corresponding speed is indicated.

Mechanical speedo cables take up the speed mostly either at the gearbox output or directly at the not driven wheel. There have been probably sporadically also speedometers that operated according to the principle of the centrifugal governor. Modern speed displays function anyway almost exclusively electronically. One or several Reed switches are prompted at the gearbox output by a revolving magnet to shifting, and the developing impulses are evaluated by a switching circuit in the speedometer or by the engine control module and transferred on a stepping motor (analogously) or on a digital display unit.