An experimental apparatus for measuring the flow of ions through membrane channels in cells. As the flow of ions amounts to a flow of current this can easily be measured. Typically a cell is placed in solution with three electrodes, two intracellular and one extracellular. One of the intracellular electrodes is used to measure voltage across the membrane relative to the extracellular electrode; the second intracellular electrode passes current into the cell. When the membrane becomes either less negative (depolarization) or more negative (hyperpolarization) these effects can be detected by the electrodes.