A potentiometer circuit is being used to find the electromotive force (e.m.f.) $E_x$ of a cell. The arrangement contains a second cell with a known e.m.f. $E_0$ and negligible internal resistance, together with a uniform resistance wire PQ of known length. $E_x$ is smaller than $E_0$. The sliding contact J may be placed at any point along the resistance wire. Which circuit arrangement is suitable for determining $E_x$?