Potentiometer Working Principle of PotentiometerPublished on 30/4/2013 & updated on 10/12/2018
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.
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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