Theory of a ferromagnetic heat engine

A ferromagnetic wheel may be made to do mechanical work if the rim is passed through a magnetic field and a source of heat. The theory of this device is developed, first when the wheel is paramagnetic, and then for the ferromagnetic case. Expressions for the efficiency, work done per revolution and mean torque are derived for the general case. Relatively high torques are possible. For an iron wheel the efficiency would be only about 0.05%, but much higher efficiencies could be attained at low temperatures in the region of 20°K using materials of low Curie points. The maximum possible efficiency is calculated to be 27.6%.