Final answer:
A potentiometer is used to accurately measure the emf or potential difference through a null measurement technique that avoids current flowing through the measuring device, by creating a potential drop along a wire.
Step-by-step explanation:
A potentiometer measures the electromotive force (emf) or potential difference by using a null measurement technique for greater accuracy, avoiding current flow through the measuring device. When using a potentiometer, a constant current is passed through a resistor, typically a long wire, which creates a steady drop in potential along the wire — known as an IR drop. By making contact at different points along the wire, a variable potential is obtained. If an unknown emf is connected in series with a galvanometer to the potentiometer and adjusted until the galvanometer reads zero, the unknown emf can be calculated as proportional to the resistance of the selected wire segment.
Null measurement techniques like using a potentiometer ensure no current flows through the measurement device, enhancing the accuracy of voltage determination.