Question

Shown in figure is a Post Office box. In order to calculate the value ofexternal resistance, it should be connected between :

• A. B and C
• B. A and D
• C. C and D
• D. B and D

Answer 1

The correct option is B A and D

A Post Office Box is used to measure an unknown resistance. It is a Wheatstone bridge with three arms P, Q and R ; while the fourth arm(S) is the unknown resistance.

P and Q are known as the ratio arms while R is known as the rheostat arm.

At balance, the current flowing through galvanometer (G) is zero and the unknown resistance S is calculated using the formula.

$S=\frac{P}{Q}R$