Question

The figure shows a potentiometer arrangement. D is the driving cell. C is the cell whose emf is to be determined. AB is the potentiometer wire and G is a galvanometer. J is a sliding contact which can touch any point on AB. Which of the following are essential conditions for obtaining balance?

• A. The emf of D must be greater than the emf of C.
• B. Either the positive terminals of both D and C or the negative terminals of both D and C must be joined to A.
• C. The positive terminals of D and C must be joined to A.
• D. The resistance of G must be less than the resistance of AB.

Answer 1

Answer: (A), (B)

The correct options are
A The emf of D must be greater than the emf of C.
B Either the positive terminals of both D and C or the negative terminals of both D and C must be joined to A.

Potentiometer is used to measure the emf of a cell using null deflection method i.e. we get C's emf when the current does not flow through the galvanometer (G) . For this to happen, C's emf= AJ's emf. The maximum limit of AJ is AB which is nothing other than D's emf. Hence, C's emf has to be less than D's emf. Moreover, the "voltage difference" measured along AJ has to be equal to that of across C. This difference could either be positive or negative. To ensure that we atleast get a null point , given we fulfill the first condition, either the positive terminals of both D and C or the negative terminals of both D and C has to be joined to A .