An experimental method of indirectly measuring the potential difference across a single mercury-electrolyte interface is suggested. By choosing a model for the moving electrolyte-mercury-electrolyte system, the maximum output voltage (which is mathematically the potential difference across a single interface) is obtained when the back end of the moving mercury drop becomes a hemisphere and the front end becomes a flat circular disc. It has been shown that the U-effect II and the U-effect III are essentially the manifestations of the same effect namely, that the output voltage is due to the distortion of the surface areas of the two ends of the moving mercury drop while the number of dipoles across each interface remains fixed. The output voltage has been found to be dependent on the velocity and the acceleration of the mercury drop and not on the relative displacement of this drop with respect to the capillary wall. The dependence of the output voltage on the acceleration of the mercury drop is opposite to that of the velocity.