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A transistor oscillator which is stable up to 150°F has been developed for obtaining electrical measurements. A reactance-modulation technique uses time rather than gain to control the frequency shift. Reactive current of constant amplitude is applied to an L-C circuit for a controlled portion of each cycle. The portion of the cycle is controlled by a bias current injected into the modulating transistor. A differential converter circuit which functions also as a dc amplifier is used with the oscillator in making voltage measurements at high impedance level. Drift of the unit with variation in temperature, vibration, and supply voltage is comparable to that of vacuum-tube multivibrators and dc-amplifier combinations.