How do Coulter counters work?

Coulter instruments operate on the principle of Electric Zone Sensing.

Electric zone sensing involves detecting a particle by measuring the electrolyte it displaces when passing through an aperture of a specific size.

The particle needs to be suspended in an electrically conductive solution which is drawn through the aperture under pressure. As the particle passes it displaces an equivalent volume of solution from the aperture space. This results in a momentary drop in the electrical current passing through the aperture.

The signal pulse generated is proportional to the displaced volume and as such gives a measurement of the particle size. Collecting a series of pulses over time and knowing the volume drawn through the aperture allows the size and density of particles to be determined.