Post Office Box

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. Eariler, it was used in post office to measure the resistance of a wire. So, it is named as post office box.

Post office box

It also works in the principle of Wheatstone bridge.

In the balanced condition,

$$\frac{\text{P}}{\text{Q}} = \frac{\text{X}}{\text{R}}$$

The box consists of three resistance arm P, Q and R.  The value of unknown resistance is determined with the help of three known resistance.

Due to the reduced application of Post office box, this topic has already been removed from the HSEB based syllabus. So, no questions will be asked from this topic anymore.

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