The electric potential differences which arise across the length of capillary tubes containing IN Perchloric acid and mercury drops are studied experimentally and theoretically for conditions of constant acceleration and constant velocity of the mercury drops relative to the capillary walls. A relatively simple theory explains many features of the dependence of the potential difference on experimental parameters. The results suggest that by coupling mechanical measurements with electrochemical measurements could yield inferfacial parameters which are difficult to determine by electro chemistry alone.