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(A) For a potentiometer to function correctly,the potential drop across the wire $AB$ must be greater than the $emf$ of the cell $C$ being measured. I

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$r > R_0$

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(A) For a potentiometer to function correctly,the potential drop across the wire $AB$ must be greater than the $emf$ of the cell $C$ being measured. If the $emf$ of $C$ is greater than the potential drop across $AB$,the galvanometer will never show zero deflection.Also,the positive terminal of the driver cell $D$ and the positive terminal of the cell $C$ must be connected to the same point $A$. If the negative terminal of $C$ is connected to $A$,the potentials will add up instead of opposing,preventing a null point.Regarding the options:$1$. $emf$ of $C > emf$ of $D$ (Option $C$) is a valid reason for no null point.$2$. Negative terminal of $C$ connected to $A$ (Option $D$) is a valid reason for no null point.$3$. $R >> R_0$ (Option $B$) can make the potential drop across $AB$ very small,potentially less than the $emf$ of $C$,which is a valid reason.$4$. The condition $r > R_0$ (Option $A$) is not a fundamental requirement for the existence of a null point in a potentiometer circuit. The null point depends on the potential balance,not on the internal resistance $r$ being smaller than the wire resistance $R_0$.

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 a reason for this?

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