Abstract: Apparatus generates a pulsed, alternating magnetic field which is coupled to magnetizable particles of submicron size dispersed in colloidal suspension in a medium. The frequency of the applied magnetic field is related to the size of the particles in such a manner as to maximize dissipative heat coupling to the medium. The differential temperature rise in the area of the medium surrounding the particles may be controlled by the concentration of the particles. Thus, defined temperature rises may be selectively produced in highly localized areas.