A description is given of a simple method of showing the variation in the M.M.F. of a single-phase alternator winding by the shadow of a vane attached to a rotating spindle.The application of the same principle to the case of polyphase windings, and the method of arriving at the position and dimensions of the vanes, are explained.An apparatus is described which is arranged to show the M.M.F. produced in the air-gap of an induction motor by the stator and rotor windings under load conditions, with the rotor either stalled or revolving.