Abstract: A test instrument for an electrochemical cell generates a current which counteracts the cell current so that it is briefly reduced to zero thereby allowing the electrode potential or cell potential to be measured free from the IR or ohmic polarization loss. A sample and hold circuit is enabled shortly after the current is interrupted to sense the voltage level of the electrode or electrochemical cell which has only experienced the voltage change due to the IR polarization losses. When cell current is restored, the IR corrected potential stored in the sample and hold circuit is compared to the control potential, and the difference between the two causes automatic adjustment of the average cell current such that the IR corrected potential of the cell is maintained despite any changes in IR polarization.