The various losses produced by flux oscillations in the unwound pole-face are considered for three main types of shoe, namely the solid ungrooved, the solid grooved and the laminated. The treatment is theoretical and includes in particular an examination of the evaluation of the amplitude of flux oscillation, the quantitative effect of harmonics, and the method of dealing with the permeability, which is a variable quantity.The losses considered are those due to eddy currents, to hysteresis and to pulsations; it is found that in normal designs only eddy-current loss has importance. The resulting equations are applied to independently published experimental results, and the calculation of a typical alternator is included showing the effect of grooving both by theory and by test. These applications confirm the general validity of the equations.