A sensitive and accurate method is described for measuring electromotive force through large resistances, up to around 100,000 megohms, or for measuring electromotive force in systems which are easily polarized. The 7 Youden and Dobroscky, Contrib. Boyce Thomson Inst. 3, 347 (1931). 8 MacInnes and Belcher, J. Am. Chern. Soc. 53, 3315 (1931).