Using a Potentiometer
If then at some point between A and B the potential will be equal toIf the potentiometer wire between A and B is uniform, then the potential at C will be proportion to the distanceWhen the potentiometer contact is at C, the potential difference across AC due toisand this is the voltage across the cell with EMFso that no current flows. We have
These equation assumes that the contacts all have zero resistance and that the potentiometer wire is uniform. These conditions can be closely approached.