What does the Coulter principle for counting cells rely on?

The Coulter principle for counting cells relies on the property that cells are non-conductive particles suspended in an electrically conductive solution, such as an isotonic saline solution. When cells pass through an aperture in the device, they displace a volume of fluid, causing changes in electrical resistance that can be measured. Specifically, the principle operates on the fact that the isotonic solution conducts electricity better than the cells do.

When a cell, which is less conductive, enters the path of the electrical field created by the Coulter counter, it creates a momentary increase in resistance as it takes up space within the conductive path. This change in resistance is proportional to the volume of the cell that has passed through the aperture, allowing the Coulter counter to detect and count the cells based on these resistance changes. Thus, the fundamental aspect of the Coulter principle is the difference in conductivity between the isotonic solution and the non-conductive cellular material.