A tachometer using the eddy-current effect has: a magnetic field generating device (11) with air gap (11.3); a rotor (10) which has a circumferential skirt (10.2) extending in the axial direction, said rotor being arranged relative to the magnetic field generating device and having its skirt so constructed that the latter essentially completely penetrates the field in the air gap continuously within the prescribed axial tolerance; one or more magnetic field measuring devices (12a, b) which measure the strength of the magnetic field at the points at which the rotor leaves &/or enters the air gap; and a measuring circuit (13) which forms from the measured values a signal which indicates the rotational speed of the rotor. The output signal is thus independent of any axial movements of the rotor resulting from thermal or other effects. <IMAGE>
