# A student uses the circuit diagram of a potentiometer as shown in the figure(a) For a steady current I passing through the potentiometer wire, he gets a null point for the cell $\varepsilon _{1}$and not for $\varepsilon _{2}$ . Give the reason for this observation and suggest how this difficulty can be resolved.(b) What is the function of resistance R used in the circuit? How will the change in its value affect the null point?(c) How can the sensitivity of the potentiometer be increased?

a)  Both $\varepsilon _{1}$  have the positive terminal connected at  A whereas negative terminal of $E_{2}$ is connected to A. So null point is not obtained for cell 2

By interchanging the terminal of $\varepsilon _{2}$, the difficulty can be resolved.

b) The resistance R protects the galvanometer by reducing the current flowing through it. As the null point is obtained for zero galvanometer reading, change in resistance R does not affect the null point reading.

c) By increasing the length of the potentiometer the sensitivity can be increased.

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