calibration apparatus for use with particle volume measuring equipment us the coulter process , which detects and analyzes the voltage pulses produced by particles passing a narrow aperture in which an electric field is maintained . the calibration apparatus includes a calibration pulse generator with pulse - shaping circuitry which produces a sequence of calibration pulses , approximately trapezoidal in shape , which are introduced into the particle measurement path , in series with the measurement aperture traversed by the particles . the generator includes precision attenuation resistors for varying the amplitude of the calibration pulses . the apparatus also provides for adjusting and maintaining a constant current through the measurement aperture and for amplifying the pulses produced by particles passing through the aperture whose amplitude is compared with the amplitude of the calibration pulses .