Post Office Box (electricity)
The boxes were used in the United Kingdom by engineers from the then General Post Office, who were responsible for UK telecommunications to trace electrical faults, i.e. to determine where a break occurred in a cable which could be several miles in length. It works on the principle of Wheatstone bridge to identify the resistance of wire connected and then by using wire resistivity and cross section calculating length of wire and thus determining where the cable had broken.
The working of P.O. Box was also a part of the IIT-JEE examination syllabus but has been removed now.
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 co-efficient.
Pairs of ratio arms are each 10, 100, 1000 ohms. Resistance arms contains a number of coils from 1 to 5000 ohms with a plug for infinite resistance.
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