in case of potentiometer emf E is directly proportional to l(length of the potentiometer wire on which galvanometer shows null deflection).
hence, E=kl, k is potential gradient;
Q. If emf of a cell is decreased, what will be the effect on position of zero deflection in a potentiometer?
Ans=k will decrease, hence, l will increase.

but if E is directly proportional to l, the as per the question l should decrease!!!