PURPOSE: To prevent a transformer from becoming large and to prevent its efficiency from being dropped by a method wherein an ultralow-frequency component or a component which is equal to it is generated and it is applied so as to offset an ultralow-frequency component flowing in the transformer. ;CONSTITUTION: An autotransformer 2 is moved back and forth, and a signal ring-modulated by a 50-Hz signal is generated. In the signal, an ultralow- frequency component is cut by a capacitor 21, the signal is supplied to a primary winding of a step-up transformer 3, and the signal is supplied to the input side of an autotransformer 22. A voltage which has been adjusted by the autotransformer 22 according to the electrostatic capacitance 11 of a cable to be inspected is supplied to individual anodes for thyristors 24 to 27 via an insulating transformer 23. When individual gates for thyristors 24 to 31 are controlled properly in this state, a signal which is used to offset the ultra-low-frequency component of the step-up transformer 3 is obtained across a capacitor 32, and the signal is supplied to the step-up transformer 3 in a state that 50-Hz waves contained in a commercial power supply have been cut by reactors 33, 34 for 50-Hz-wave suppression.;COPYRIGHT: (C)1994,JPO
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