A Potentiometer is a electrical device used to measure unknown voltage by comparing it with a known voltage. It comprises of a slide wire of uniform cross-section and hence uniform resistance along its length. A calibrated scaling cm and fraction of cm is placed along the slide wire.
Before using the potentiometer it should be standardized. It is the process of adjusting working current so as to match the voltage drop across a portion of sliding wire against standard cell, whose voltage is constant at 1.0186 Volts at all times. The purpose of this standardization is to make the potentiometer “direct reading”.
Standardization of a potentiometer is done by connecting a standard-cell to the terminals marked “Standard-Cell”, Galvanometer and Battery to their respective terminals. Potential dials are set to the effective voltage of the standard cell. Now by pressing “Standardize” key and adjusting battery Rheostats (coarse and fine) balance is obtained. The galvanometer should show no deflection with standardized key pressed and zero series resistance. Then the potentiometer is said to be “standardized”.
After standardization unknown voltage is applied to the potentiometer. By adjusting the dial resistor and slide-wire, “null” balance is obtained. Under balanced conditions, the readings of slide-wire and dial resistor together gives the value of unknown voltage.