# Potentiometer Working Principle of Potentiometer

Published on 30/4/2013 & updated on 10/12/2018
This is a very basic instrument used for comparing emf two cells and for calibrating ammeter, voltmeter and watt-meter. The basic working principle of potentiometer is quite simple. Suppose we have connected two batteries in parallel through a galvanometer. The negative battery terminals are connected together and positive battery terminals are also connected together through a galvanometer as shown in the figure below. Here, if the electric potential of both battery cells is exactly same, there is no circulating current in the circuit and hence the galvanometer shows null deflection. The working principle of potentiometer depends upon this phenomenon.

Now let's think about another circuit, where a battery is connected across a resistor via a switch and a rheostat as shown in the figure below. The resistor has the uniform electrical resistance per unit length throughout its length. Hence, voltage drop per unit length of the resistor is equal throughout its length. Suppose, by adjusting the rheostat we get v volt voltage drop appearing per unit length of the resistor. Now, the positive terminal of a standard cell is connected to point A on the resistor and negative terminal of the same is connected with a galvanometer. The other end of the galvanometer is in contact with the resistor via a sliding contact as shown in the figure above. By adjusting this sliding end, a point like B is found where there is no current through the galvanometer, hence no deflection in the galvanometer.

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Potentiometer Working Principle of Potentiometer

That means, emf of the standard cell is just balanced by the voltage appearing in the resistor across point A and B. Now if the distance between point A and B is L, then we can write emf of standard cell E = Lv volt.

This is how a potentiometer measures the voltage between two points (here between A and B) without taking any current component from the circuit. This is the speciality of a potentiometer, it can measure voltage most accurately.

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