The post office box was a
Wheatstone bridge
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to prov ...
–style testing device with pegs and spring arms to close electrical circuits and measure properties of the circuit under test.
Resistance measurement
The boxes were used in the United Kingdom by engineers from then
General Post Office
The General Post Office (GPO) was the state postal system and telecommunications carrier of the United Kingdom until 1969. Established in England in the 17th century, the GPO was a state monopoly covering the dispatch of items from a specific ...
, 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 were common pieces of scientific apparatus in the UK
O-Level and
A-Level
The A-level (Advanced Level) is a subject-based qualification conferred as part of the General Certificate of Education, as well as a school leaving qualification offered by the educational bodies in the United Kingdom and the educational ...
schools public examination
physics
Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
syllabus in the 1960s.
Construction
A typical post office box is in a wooden box with a hinged lid and a metal or
bakelite
Bakelite ( ), formally , is a thermosetting polymer, thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. The first plastic made from synthetic components, it was developed by Belgian chemist ...
panel showing circuit connections. Coils of wire are wound non-inductively, mounted in the body of the box, and have a negligible temperature coefficient.
Pairs of ratio arms are each 5 10 20
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (1 ...
s. Resistance arms contains a number of coils from 1 to 5000 ohms with a plug for infinite resistance.
See also
*
References
Electrical meters
Electronic test equipment
Historical scientific instruments
Scientific instruments
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