 Wallace H. Coulter

Wallace H. Coulter was an engineer and inventor who developed electrical counting of suspended microscopic particles, the principle behind the Coulter counter. His patent application was filed on 27 August 1949 and granted as US2656508 on 20 October 1953; these are different events in the history of the apparatus.^[ref]

The apparatus passes an electrically conducting suspension through a small aperture between electrodes. A particle crossing that restricted path changes its electrical behavior and produces a detectable pulse. Counting pulses in a measured sample volume estimates the number of particles; known dilution connects that estimate to the original sample. The setup replaces the need to identify and count every particle visually with a repeatable electrical measurement.^[ref]

For example, 2500 accepted pulses in $50\mu \mathrm{L}$ after a hundredfold dilution imply $2500\times 100/50=5000$ counted particles per microlitre in the original suspension. This synthetic example assumes that particles are well mixed and individually resolved. Two particles passing together can be confused with one event, so a counter requires control of sample concentration as well as electronic sensitivity.