Several papers have been published recently on the subject of transductors; these have been concerned largely with transductors operated from voltage sources, and particularly with their performance when applied as magnetic amplifiers. When transductors are operated from current sources, although their characteristics do not differ greatly from those of voltage-operated transductors, their field of application is widened. The object of the present paper is to describe some of the many ways in which transductors may be used (not necessarily as amplifiers) to give to simple relay elements some of the special characteristics required in protective-gear work.In order to illustrate more clearly the principles involved, especially for protective-gear engineers who may not be familiar with transductors, some basic transductor characteristics (described elsewhere in greater detail) are included. Special control arrangements and constructional features are next considered; and the main body of the paper contains descriptions of a number of practical applications of transductors as protective relays.The transductor is shown to be a very versatile component for protective systems.