Potentiometer

Potentiometer, device used to obtain a precise measure of the electromotive force (emf), or voltage, of an electrical cell. It employs a special circuit incorporating a variable resistor (rheostat) and a galvanometer. Using a standard cell of known emf, the rheostat is adjusted until no current flows in the circuit, as indicated by the galvanometer. The cell of unknown emf is then placed in the circuit instead of the standard cell, and the rheostat is adjusted again so that no current flows. The difference in the 2 settings of the rheostat indicates the emf of the unknown cell. The potentiometer can be used to calibrate electrical instruments and as a variable resistor. The volume control in a radio is a type of potentiometer.