The fundamental theory of coupled circuits with distributed constants is reviewed and applied to the problem of designing an oscillator for use in parallel line measurements. An oscillator having a continuously variable frequency range from 75 to 500 megacycles, and which fulfills the theoretical requirements which permit the e.m.f. induced by it in a parallel line to be represented by point generators, is described. Its frequency characteristic is displayed, and its application to parallel line measurements is discussed briefly.