A sinusoidal alternating voltage is generated and applied to the input of an unknown network; the output voltage of the network is displayed on the Argand diagram reproduced on the screen of a cathode-ray tube. As the frequency is varied the circle diagram of the network is traced on the screen and resonances are indicated by loops in the trace. The frequency range is 5-215 kc/s. There are facilities for frequency calibration; a "dotted line" trace can be produced over a limited frequency range, each dot representing an equal increment in frequency. Circuit details are given. The chief applications of the present design are described - the assessment of electromechanical transducers and as a bridge detector in electromechanical measurements of elastic constants. Numerous electrical circuit applications would also be possible.