Post Office Box: The resistances of resistance box that are used as three arms of a Wheatstone and by inserting an unknown resistance in the fourth arm its resistance can be measured by applying Wheatstone bridge principle is called post office box.
It is an electrical device that is used to find the resistance of the object. It can also compare the resistance of the two wires. Earlier, it was used in the post office to measure the resistance of a wire. So, it is named as post office box.
A typical post office box is in a wooden box with a hinged lid and a metal or bakelite panel showing circuit connections. Coils of wire are wound non-inductively, mounted in the body of the box, and have a negligible temperature coefficient. Pairs of ratio arms are each 10, 100, 1000 ohms. A resistance arm contains a number of coils from 1 to 5000 ohms with a plug for infinite resistance.