Abstract: Apparatus for producing an indication of the value of an unknown capacitance or inductance, which employs a variable frequency oscillator producing a sinusoidal output which is fed through a resonant circuit comprising a reactive circuit element of known value selected to attain resonance with the unknown reactor within the oscillator frequency range. A phase detector measures the phase shift between oscillator output voltage and output current, and its direction and supplies a phase shift signal specifying this information to a controller which supplies a frequency control signal to the oscillator. The controller varies the frequency control signal responsive to the phase shift signal to eventually reduce phase shift to 0.degree. or other predetermined value indicating the resonance condition in the circuit.