A suspension of blood cells in a fixed volume of an electrolyte solution flows through an aperture (60- 80 µm). Constant electric current is applied between platinum electrodes on either side of the aperture. Since each cell is a relative nonconductor, the passage of each cell through the aperture causes a momentary decrease in conductance and a resultant voltage spike. These pulses can be counted and sized electronically. The number of pulses can be expressed as a concentration or a blood cell count. Since the magnitude of each cell is generally proportional to the volume of displaced electrolyte, electronic averaging of the pulse heights in the red cell channel is employed as a direct measurement of the MCV of erythrocytes.