Magnetic circuit that has (a) a source of magnetic flux which includes an electromagnet or one or more permanent magnets, (b) at least two oppositely polarisable pole extension bodies associated with the magnetic flux source, the bodies being disc, wheel, roller or similarly shaped with an outer circumferential surface and held rotatable about respective axes of rotation, and (c) a ferromagnetic counter body which is arranged to cooperate with the pole extension bodies such as to provide an external flux path for the magnetic flux when in magnetic proximity or contact with the circumferential surface of the pole extension bodies, which is characterised in that the magnetic flux source is held stationary relative to the rotatable pole extension bodies.