Experience with experimental techniques for the study of damping and hysteresis in thin films has suggested that many of the difficulties and complications encountered might be avoided by the direct approach of measuring the heat generated by the irreversible processes which are of interest. This suggestion has led to the development of a simple and reliable microcalorimetric apparatus with which accurate loss determinations may be made over a frequency range of about 100 cps to 100 Mc. The loss detector employed is a thin-film thermocouple which developes an emf proportional to the total heat flux emerging from the ferromagnetic sample. The zero drift of the heat-detecting system is eliminated, and the overall noise level greatly reduced by imposing a low-frequency square-wave modulation on the magnetic losses, amplifying the resultant ac component of the thermal emf with a narrow-band amplifier, and rectifying the output by means of a commutator phase-synchronized with the modulator. It appears that losses as small as 106 erg/sec per cc of sample volume are readily detectable. The thermal method thus has a degree of sensitivity which compares favorably with that of other experimental techniques, and has the added advantage of providing a direct measure of energy loss. © 1960 The American Institute of Physics.