Potentiometer (measuring instrument)

A potentiometer, in the context of measuring instruments, is a type of electrical instrument used to measure an unknown voltage by comparing it with a known reference voltage. The principle behind a potentiometer relies on creating a voltage divider that is precisely controlled and used to null or balance against the unknown voltage.

Unlike voltmeters, which draw current from the circuit being measured and thus can alter the circuit's performance, a potentiometer ideally draws no current from the unknown voltage source at the point of measurement. This "null method" provides a highly accurate measurement, especially for low-voltage sources or circuits where minimal disturbance is crucial.

The classic form of a potentiometer consists of a long resistive wire with a uniform resistance per unit length. A known voltage is applied across this wire, creating a linear voltage gradient. The unknown voltage is then connected in opposition to a portion of the voltage drop across the resistive wire. A galvanometer is used as a null detector. By adjusting the position of a sliding contact (wiper) along the resistive wire, the point is found where the galvanometer reads zero, indicating that the voltage drop across that portion of the wire exactly matches the unknown voltage. The unknown voltage can then be calculated based on the length of the wire segment and the known applied voltage.

Modern potentiometers may employ electronic circuits and digital displays, but the fundamental principle of voltage comparison remains the same. Potentiometers are often used in applications requiring high accuracy and precision voltage measurements, such as calibration of other instruments, measurement of standard cell voltages, and in precision laboratory settings. They are distinct from potentiometers used as variable resistors in electronic circuits. The key difference lies in their application: a measuring instrument potentiometer is primarily used for accurate voltage measurement through a null method, while a variable resistor potentiometer is used for voltage division or resistance adjustment within a circuit.