This paper describes briefly the construction of typical homopolar and heteropolar inductor alternators, and gives a theoretical basis of design which has produced satisfactory results in practice.It is shown how the ratio of alternating flux to total flux can be calculated in the normal case of rectangular rotor slots and in the special case where the rotor teeth and slots are designed to give a sinusoidal alternating flux wave.In discussing the main types of armature windings it is shown how by maintaining a certain relationship between the number of rotor slots and the number of phases, a single-phase winding may be connected to give a polyphase supply.In the derivation of the armature-reaction formula it is shown that some types of windings cause less armature reaction than others, and also that, except in the case of a true polyphase winding, an efficient damping system is necessary.Methods of predetermining the field current on open-circuit and on load are indicated; the necessity of taking precautions to avoid dangerous bearing currents in the homopolar alternator is discussed, and the paper concludes with an investigation of the losses in these alternators.