Abstract: A cell for measuring the electrical resistivity of a liquid contains four aligned and linearly spaced electrodes. A current source is connected to the end electrodes and an amplifier is connected directly to the inner electrodes. The amplifier has a high input impedance characteristic sufficient to limit the current carried by the inner electrodes to a small value which renders it essentially independent of any fouling of the inner electrodes. The amplifier is connected to the current source to control the current output thereof so as to maintain a constant predetermined voltage drop across the inner electrodes whereby the output of the amplifier is independent of any IR drop due to the fouling of the outer electrodes. A reference and a measuring cell may each be connected as above with the high input impedance amplifier of the measuring cell controlling the current sources for both cells and the corresponding amplifier of the reference cell driving a suitable output indicator.