A high-speed device for particle size analysis designed by W.H. Coulter and now made by Coulter Electronics, Inc., Chicago. A suspension of the particles flows through a 759 small aperture having an immersed electrode on either side with particle concentration such that the particles traverse the aperture substantially one at a time. Each particle, as it passes, displaces electrolyte within the aperture, momentarily changing the resistance between the electrodes and producing a voltage pulse of magnitude proportional to practical volume. The resultant series of pulses is electronically amplified, scaled, and counted.