In order to clamp the voltage, to correct for solution resistance, and to record the short-circuit current (SCC) automatically in inexcitable biological membranes, three independent units are usually needed: a voltage control unit, a subtracting amplifier, and a recorder. A circuit has been devised in such a manner that one servo recorder performs these three functions. It consists of a constant current supply with a network including a ten-turn potentiometer (10-T-P) which controls and makes the amount of current delivered to the membrane linear to the position of the 10-T-P slide arm. When the 10-T-P is coupled to the motor shaft of the recorder, the recorder's pen position is also linearly related to the current circulating through the membrane. By feeding the recorder's input with the potential difference (PD) of the membrane, the balance is achieved when the PD is brought to zero (or to any other desired value). Simultaneously, the pen records the amount of current necessary to obtain the balance. To correct for the resistance of the solution, the polarity of the built-in feedback voltage network of the recorder is inverted. Then, an additional negative voltage, proportional to the SCC, is automatically added to the input. If correction for the solution resistance is not desired, the built-in feedback network is left disconnected. The balance error, dead band, and time response depend mainly upon the characteristics of the recorder utilized.