In a paper on Dynamo Electric Machinery by Dr. John Hopkinson ('Phil. Trans.,' 1896), the suggestion is made that the value of the hysteresis of the iron core of a rotating dynamo armature need not be identical with the value obtained when the magnetising force is reversed by passing through a zero value. It was subsequently pointed out by Mr. Swinburne that as a necessary deduction from Professor Ewing’s molecular theory of magnetism, the hysteresis of iron in a rotating field, or of iron rotating in a constant field, should show a distinct diminution in value below that in an alternating field, when the magnetic condition of the iron approaches saturation. According to Ewing’s theory hysteresis is due to the formation of stable magnetic combinations between adjacent molecules which tend to resist any movement of the molecular magnets caused by change of direction or magnitude in the magnetising force. On the breaking up of these combinations by such a change in the magnetising force, new arrangements are formed, and the potential energy of position is transformed into kinetic energy of partial rotational movement round the fixed axis of the molecule, which is damped out with or without oscillations above the axis of rotation. It has been further suggested that the damping process may be due to eddy currents induced by the movement of the magnets, but the precise nature of these eddy currents, or the extent to which other retarding influences akin to mechanical friction or viscosity may act has not been determined.