Measurement of Potential:

The electromotive force developed by a galvanic cell cannot be measured accurately by placing a simple dc voltmeter across the electrodes as shown in Fig. 1.3 of Unit 1, because a significant current is required for operation of the meter.  Since current is drawn from the cell, a difference in concentrations of the reacting species will take place which leads top a change in the cell voltage. An additional factor is the development of ohmic potential drop due to the internal resistance of the cell; this opposes the potential due to the two electrodes of a galvanic cell. A truly important value for the output of a cell potential can be attained only if the measurement is made with a negligible passage of current.  A potentiometer is one category of instrument that meets this specification.