To obtain an apparatus for measuring variation in the capacitance of dual capacitor by converting first and second half-wave current signals, respectively, into first and second frequency signals and then calculating the ratio of capacitance between first and second capacitors from the frequencies thus converted.

The measuring circuit 200 comprises an AC power supply 140 generating an AC voltage, an analog switching means 210, first and second current/frequency converters 220, 225, and a microcontroller 250. A voltage is fed to the common electrode 110 of first and second capacitors and first and second AC current signals are generated at the independent electrodes 132, 134 of the first and second capacitors. The analog switching means 210 converts the first and second AC current signals, respectively, into first and second half-wave current signals. The converters 220, 225 receive the first and second half-wave current signals and convert them into first and second frequency signals having respective frequencies. An ROM program in the microcontroller 250 calculates the ratio of capacitance between the first and second capacitors using respective frequencies.