Question

What is a post office box? How is the· unknown resistance measured using a post office box?

Answer 1

A post office box (PO Box) is a practical form of a Wheatstone bridge, as shown in the figure.

It has three arms: P, Q, and R. These three arms' resistance can be adjusted. The resistances in the two ratio arms P and Q are 10 ohm, 100 ohm, and 1000 ohm, respectively. Resistances range from 1 ohm to 5000 ohm in the third arm R. The fourth resistance is the unknown resistance X. K₁ and K₂. are the two tap keys. Resistances in arms P and Q are set to the required ratio. The resistance in the arm R is set so that the galvanometer does not deflect.

If L is the length of the wire and r is its radius then the specific resistance of the material of the wire is given by

`ρ = (Xpir^2)/L`