PURPOSE:To automatically measure the electrostatic capacity and loss angle of a condenser and a cable thereby to spare the inspecting and quality managing business by providing an automatic tuning circuit. CONSTITUTION:A supply voltage E is fed through a phase shifting circuit PS to a phase detecting circuit PD. If the resistance Rp of a bridge circuit, which is composed of a test capacitor A, a standard capacitor Cs, a standard resistor R and an optically variable resistor Rp, is increased, the output of a DC amplifier DCamp is decreased so that the resistance Rp takes a preset value at a preset time and so that the output of the DCamp becomes constant. Thus, signals are fed from a differential circuit D to hold circuits PH1 to PH3 of each portion so that the peak value of AC voltage at each point of the bridge circuit is held. On the basis of these data, in analog dividers D1 and D2, a DC voltage proportional to tan delta and the electrostatic capacity C is obtained and subjected to A/D conversion to automatically display at DD1 and DD2.
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