A toroidal conductivity sensor consists of two wire wound toroids encased in a plastic body. When an alternating voltage is applied to one of the toroids, it induces an ionic current in the solution surrounding the sensor. The ionic current, in turn, induces an electronic current in the second toroid, which the analyzer measures. The current in the second toroid depends on the number of turns of wire on each toroid and on the conductance of the solution. The conductance depends on the total ion concentration (the conductivity) and the area and length of solution the ionic current flows through. The physical dimensions of the sensor determine the dimensions of the current path.
Because the purpose of the measurement is to get information about the total concentration of ions in solution, i.e., the conductivity, the effect of sensor dimensions and windings has to be accounted for. The correction factor is the cell constant. Conductivity is equal to the measured conductance multiplied by the cell constant.