This paper reviews the various types of alternators available for the generation of frequencies up to 50 000 c/s, and shows that for the majority of applications the modern heteropolar inductor alternator is the most suitable machine. It demonstrates that this alternator is superior to the older homopolar and heteropolar types, owing, in the case of machines required for frequencies of 400 to 3 600 c/s, to a system of field and armature windings which substantially reduces the number of armature conductors required to generate a given e.m.f. For frequencies above 3 600 c/s this superiority is due to a combination of these windings with a stator slotting arrangement which permits, for a given frequency and speed of rotation, a large reduction in the number of wound stator slots. The electrical characteristics and mechanical construction of these alternators are discussed, together with their application to the melting of metals and surface-hardening of steel. An appendix gives a brief mathematical treatment of the theory of their novel features.