Conductivity and resistivity are measured by applying voltage between electrodes immersed in a solution to determine the size of current that flows through the solution. At this time, when the conductivity of a solution goes up, the current value does not increase linearly, rather it increases along a gradual curve due to the impact of polarization*. With the 4-electrode method, two electrodes for measuring voltage are placed between the two electrodes used to measure current. Polarization occurs on the outer electrode used to measure current, so along with measuring current, adjusting based on the voltage between the two electrodes for measuring voltage makes it possible to make accurate measurements even in solutions with high conductivity.
* Polarization is the phenomenon in which, during conductivity measurements, the charge becomes uneven at the interface between the electrode and the sample, creating an electrical barrier (resistance).When measuring high conductivity, the resistance of the sample is small, resulting in significant measurement errors due to resistance arising from polarization.
