A circuit, which is useful to provide data needed to measure the electrical opacity of a particle, for instance a blood cell, passing through a Coulter type transducer, includes a current source for providing a conventional d.c. current through the sensing aperture of the transducer, as well as an oscillator for providing a high frequency current through the aperture. The oscillator includes an active device and a resonant circuit, and the aperture is coupled in parallel with the resonant circuit of the oscillator. The oscillator can be any type of oscillator, such as a Hartley oscillator. As a particle passes through the aperture, the resistance of the aperture increases, which in turn, increases the Q of the oscillator circuit, whereby the oscillator output signal is amplitude modulated in accordance with the increased Q. This amplitude modulated signal can be detected to provide a value based on the high frequency reactance of the particle being detected. The change in d.c. resistance due to a particle passing through the aperture is detected in a conventional manner. The detected amplitudes of the a.c. and d.c. signals then can be compared to determine the opacity of each particle. Multiple oscillators of different frequencies can be coupled in parallel with the transducer through a coupling circuit, which connects each oscillator to the transducer through a low impedance path and isolates each oscillator from signals having frequencies of the other oscillators.