what is meant by a post office box? and menntion its proper uses.

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 = (P/Q) R