The measured e.m.f. of some saturated Weston cadmium cells is raised while an alternating current of a few microamperes is passing through them. The rise is approximately proportional to the square of the amplitude of the alternating current; over the frequency range studied (50-300 Hz) it diminishes as the frequency is increased. The magnitude of this effect is correlated with the internal resistance and with the age of the cells. Attention is drawn to the relevance of these results to the design of mains-driven current supplies for precision potentiometers.