Principle: If constant current is flowing through a wire of uniform area of cross-section at constant temperature, the potential drop across- any portion of wire is directly proportional to the length of that portion
i.e., V ∝ l
Method: (i) Initially key K is closed and a potential difference is applied across the wire AB. Now rheostat (Rh) is so adjusted that on touching the jockey J at ends A and B of potentiometer wire, the deflection in the galvanometer is on both sides. Suppose that in this position the potential gradient on the wire is k.
(ii) Now key K1 is kept open and the position of null deflection is obtained by sliding and pressing the jockey on the wire. Let this position be P1 and AP1 = l1. In this situation the cell is in open circuit, therefore the terminal potential difference will be equal to the emf of cell,
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i.e., emf ε = kl1 ...(i)
(iii) Now a suitable resistance R is taken in the resistance box and key K1 is closed. Again, the position of null point is obtained on the wire by using jockey J. Let this position on wire be P2 and AP2 = l2 .In this situation the cell is in closed circuit, therefore the terminal potential difference (V) of cell will be equal to the potential difference across external resistance R, i.e.,
![](https://www.sarthaks.com/?qa=blob&qa_blobid=5157626727219111722)