Potentiometer null method for measuring unknown emf

Steps:

• Connect known emf battery across uniform wire XY to create potential gradient.
• Link unknown cell in opposition to battery, with galvanometer from cell terminal to jockey on wire.
• Adjust jockey for null galvanometer deflection, ensuring no current flows through unknown cell.
• Calculate unknown emf as E = (l/L) × known emf, where l is balancing length and L is wire length.

Why B is correct:

• It opposes unknown emf to potential drop along wire, allowing null balance per potentiometer principle where E_unknown = I × ρ × l (ρ is resistance per unit length, I is current).

Why the others are wrong:

• A: Galvanometer directly across wire, measures resistance not emf.
• C: Unknown cell powers wire, reverses setup and draws current from it.
• D: Series connection of cells bypasses null method, no balancing point.

Final answer: B