Question

In the given potentiometer circuit, the resistance of the potentiometer wire $AB$ is $R_0$. $C$ is a cell of internal resistance $r$. The galvanometer $G$ does not give zero deflection for any position of the jockey $J$. Which of the following cannot be the reason for this?

• A. $r>R_0$
• B. $R>>R_0$
• C. emf of $C$ > emf of $D$
• D. the negative terminal of $C$ is connected to $A$

Answer 1

The correct option is A $r>R_0$

For potentiometer to measure the internal resistance of cell it should be connected across point A of wire and galvanometer, with positive terminal to point A and negative terminal to galvanometer. Emf of E should not exceed emf of driver cell. Also resistance connected in series with cell in galvanometer circuit should not far more than resistance of wire. If reverses all the above conditions jockey will not show deflection.
Also, $r=R(\frac{l_1}{l_2}-1)$ . Hence, $r$ does not depends on resistance of potentiometer wire.