The invention consists of a toothed wheel which in conjunction with a fixed sensor measures the revolutions or the angular velocity of a rotating component. The toothed wheel is constructed as a coil spring mounted on a circular ring-shaped mounting device and moved around a pivot with the rotation motion to be measured. The coil spring consists of an infinite helix the windings of which are of ferromagnetic material. Since the coil spring is moved past the fixed sensor, the change of the magnetic field in the coil of the sensor causes the generation of an alternating voltage the frequency of which is substantially proportional to the angular velocity of the toothed wheel.