Resistance (Coulter) Particle Counters work on the principle of aperture technology, which is also called Coulter Principle. As is illustrated in the picture below, a tube with an aperture is immersed in a container of electrolyte with suspended particles. A cathode inside the aperture tube and an anode outside the aperture but inside the electrolyte allows a flow of current through the aperture when electricity is supplied. A negative air pressure inside the aperture tube draws the electrolyte outside into the tube, together with the suspended particles. The cross sectional area of the aperture gets smaller as particles pass through it, which is followed by an increased resistance between the two electrodes. The change in the resistance results in a voltage pulse. Assuming a constant current is supplied, the pulse height is proportional to the volume of the particle passing through the aperture. Therefore, by measuring each pulse height, such resistance counters can calculate the size of each particle and hence output the size distribution of all particles analyzed.