The calorimeter described was designed for the purpose of measuring the power loss in magnetic ring specimens under non-incremental audio-frequency excitation, with the aim of checking results of measurements by an Owen bridge under conditions involving appreciable nonlinear distortion.Reasons for the choice of calorimetric method are given, and the use of the calorimeter as a transfer instrument is considered.The use of the Owen bridge for iron-loss measurements is discussed, and results are given of measurements made on ring specimens built up of silicon-steel stampings of two different thicknesses, for values of flux density up to 10 kilogauss and of frequency up to 10 kc/s.