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The post office box works on the principle of:
A) Potentiometer
B) Matter waves
C) Ampere’s law
D) Principle of wheatstone bridge

Last updated date: 13th Jun 2024
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Hint: Post office box is a wheatstone bridge style testing device with pegs and spring arms in order to close the electrical circuit to measure the properties of the circuit under test. Basically, a post office box is a measuring device. Using the above concept we will find out the principle on which the post office box works.

Complete solution:
Post office box is a typical testing device which consists of the combination of resistances. Generally, a post office box is kept in a wooden box with a hinged lid. Physicists named the post office box because it is fabricated to find the resistance of electric cables and telegraph wires(telecommunication purposes), also used by post offices to figure out the resistance of the transmission lines laid for communication objectives. Specific resistance, internal resistance of the cell, resistance of the galvanometer can also be measured by post office box.
Post office box consists of four arms having three arms with known resistances and the fourth arm having unknown resistance which we need to determine, the principle of post office box is somewhat like that of wheatstone bridge(Wheat stone bridge is also used for measuring resistance by balancing resistance of the arms of the bridge).

Thus, option (D) is correct.

Note: The post office box was used earlier in telecommunications to trace the electrical faults; to determine where a break occurred in a cable which could have been very long in length which is difficult to detect without any measuring instrument. Post office box measures the reading gives the value of the measured quantity that could be compared with the actual quantity for fault detection.