A variometer is described, of such construction that it may be used as the tank inductance in a Hartley oscillator, maintaining a constant excitation ratio over a wide frequency range. When the circuit is arranged to keep the inductance‐capacitance ratio constant also, the efficiency and power output are correspondingly uniform. Although the problem, in the case illustrated, is complicated by the design of the oscillator for high plate voltages, the basic design is equally applicable to oscillators of lower power and much wider frequency range.