Longman Dictionary of Contemporary English
Wikipedia
The Post Office Box was a Wheatstone bridge-style testing device with pegs and spring arms to close electrical circuits and measure properties of the circuit under test.
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.
Post Office Boxes also were common pieces of scientific apparatus in the UK O-Level and A-Level schools public examination Physics syllabus in the 1960s.
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.
Usage examples of "post office box".
You'd used one of your father's envelopes, on the flap of which appeared his address: Post Office Box 69, Southern District, Eulalia, N.
Don't send me a single dime to post office box eight three nine, Christian City, Louisiana 70539.
Just as anyone can rent a post office box, like the one Brianna wrote to in Wales.
He had memorized the six-page treatise on American security procedures which had been sent to him care of a post office box drop, along with his specific instructions.