The maximum sensibility of a moving coil vibration galvanometer as a voltage detector is obtained when the flux through it is so adjusted that the back E.M.F. of the coil is equal to its CR drop; then the back E.M.F. is equal to half the applied voltage, and the current is equal to V/2R and is in phase with the applied voltage.Increases of current sensibility of about 30 per cent. at 200 similar and 40 per cent. at 1,000 similar were obtained on running the instrument in a vacuum, thus showing that a large part of the mechanical work produced was used in overcoming the molecular friction of the system. Tables and curves are given showing the variations of voltage and current sensibility with alteration of flux, &c.