present day his principle has remained in use. At a glance, a potentiostat measures the potential difference between the working and the reference electrode, applies the current through the counter electrode and measures the current as an voltage drop over a series resistor ( in Fig. 1). The control amplifier (CA) is responsible for maintaining the voltage between the reference and the working electrode as closely as possible to the voltage of the input source . It adjusts its output to automatically control the cell current so that a condition of equilibrium is satisfied. The theory of operation is best understood using the equations below. Prior to observing the following equations, one may note that, from an electrical point of view, the electrochemical cell and the current measurement resistor may be regarded as two impedances (Fig. 2). includes in series with the interfacial impedance of the counter electrode and the solution resistance between the counter and the reference. represents the interfacial impedance of the working electrode in series with the solution resistance between the working and the reference electrodes. The role of the control amplifier is to amplify the potential difference between the positive (or noninverting) input and the negative (or inverting) input. This may be translated mathematically into the following equation: . (1) where is the amplification factor of the CA. At this point the assumption may be made that a negligible amount of current is flowing through the reference electrode. This correlates to physical phenomenon since the reference electrode is connected to a high impedance electrometer. Thus, the cell current may be described in two ways: (2) and . (3) Combining Eqs. (2) and (3) yields Eq. (4): (4) where is the fraction of the output voltage of the control amplifier returned to its negative input; namely the feedback factor: . Combining Eqs. (1) and (4) yields Eq. (6): . (6) When the quantity becomes very large with respect to one, Eq. (6) reduces to Eq. (7), which is one of the negative feedback equations: . (7) Eq. (7) proves that the control amplifier works to keep the voltage between the reference and the working close to the input source voltage. Software control Replacing the CA, a control algorithm can maintain a constant voltage between the reference electrode and the working electrode. This algorithm is based on the rule of proportion: . (8) is the last measured cell