An electro-magnetic brake which is switched between an operative braking position and an inoperative position by transient pulses of current. It is held passively in the operative and inoperative positions with little or no current. This prevents the generation of heat, which would affect the accuracy of measurements where the brake is used in dimensional measuring equipment. In one form, permanent and semi-permanent magnets (54,56) are arranged in parallel,and the magnetic polarity of the arrangement is flipped by transient current pulses in a surrounding coil (58). A capacitor may provide a reservoir of current to activate the brake in the event of a power supply failure.