The principle on which the electric differential is based can be employed in a number of different ways, leading to low-power applications of an instrument or indicator type, which have been discussed earlier. The principle can be applied on the one hand, for example, to phase-angle meters, synchroscopes, phase-sequence indicators and, on the other hand, to devices which supply more than token loads and for which power requirements must be evaluated in greater detail, and these have not hitherto been described. When the stator of a salient-pole machine is doubly fed in a particular manner, the shaft rotates at a frequency equal to one half the difference between the angular frequencies of the two polyphase systems employed—the feature from which the term ‘differential’ derives. The present paper examines the behaviour of this device and establishes simple theoretical principles which lead to stationary- or quasistationary-performance equations on which further developments can be based.