A potentiometer is an instrument for measuring the potential (or voltage) in a circuit. The instrument taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The sliding contact or wiper of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding contact and the unknown potential. The deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact.
Before the introduction of the moving coil and digital volt meters, potentiometers were used in measuring voltage, hence the '-meter' part of their name. This null balance method is still important in electrical metrology and standards work. The null-balance principle of measurement is also used in other areas of electronics.
The potentiometer used for measurement is a type of bridge circuit for measuring voltages by comparison between a small fraction of the voltage which could be precisely measured, then balancing the two circuits to get null current flow which could be precisely measured. Measurement potentiometers are divided into four main classes listed below.