Potentiometer null point shifts with changes to potential gradient

Steps:

• Balance condition: cell EMF ε equals potential drop to null point, ε = k l, where k is potential gradient, l is balance length.
• k proportional to current I through wire XY; I = driving EMF / (R + R_XY).
• Decreasing R increases I, raising k.
• Higher k for E1 reduces its balance length l' = E1 / k versus original, creating new null position P.

Why D is correct:

• Decreasing R boosts current I per Ohm's law, increasing k and shifting E1's null point to a shorter length, a new position.

Why the others are wrong:

• A. Lower XY resistance reduces k (less PD per length for same I), lengthening null point for E1.
• B. Increasing E1 raises ε, lengthening null point without circuit change.
• C. Decreasing E2 affects only its own balance, not E1's position.

Final answer: D