Editing Twisted pair (section)

==History==
[[File:Wire Transposition.svg|thumb|Wire transposition on top of pole]]

The earliest telephones used telegraph lines which were [[Earth-return telegraph|single-wire earth return]] circuits. In the 1880s electric [[tram]]s were installed in many cities, which induced noise into these circuits. In some countries, the tram companies were held responsible for disruption to existing telegraph lines and had to pay for remedial work.{{efn|In Cape Town for instance, a balancing conductor was installed from the telegraph office through the streets and six miles out to sea to fix interference to the [[submarine telegraph cable]] from [[Luanda]].<ref>Trotter, A.P., [https://web.archive.org/web/20190728174443/https://ieeexplore.ieee.org/document/5309139 "Disturbance of submarine cable working by electric tramways"], ''Journal of the Institution of Electrical Engineers'', vol. 26, iss. 130, pp.&nbsp;501–514, July 1897.</ref>}} For new installations, however, it was necessary to protect against existing trams from the outset. Interference on telephone lines is even more disruptive than it is on telegraph lines. Telephone companies converted to [[balanced circuit]]s, which had the incidental benefit of reducing [[attenuation]], hence increasing range.

As electrical power distribution became more commonplace, this measure proved inadequate. Two wires, strung on either side of cross bars on [[utility pole]]s, shared the route with electrical [[power line]]s. Within a few years, the growing use of electricity again brought an increase of interference, so engineers devised a method called [[wire transposition]], to cancel out the interference.

In wire transposition, the wires exchange position once every several poles. In this way, the two wires would receive similar [[Electromagnetic interference|EMI]] from power lines. This represented an early implementation of twisting, with a twist rate of about four twists per [[kilometer]], or six per [[mile]]. Such open-wire balanced lines with periodic transpositions still survive today in some rural areas.

Twisted-pair cabling was invented by [[Alexander Graham Bell]] in 1881.<ref>
{{cite patent
 | inventor1-last = Bell
 | inventor1-first = Alexander Graham
 | title = Telephone-circuit
 | issue-date = 1881
 | patent-number = 244426
 | country-code = US
}}. See also TIFF format scans for [http://patimg1.uspto.gov/.piw?Docid=00244426&idkey=NONE USPTO 00244426]
</ref> By 1900, the entire American telephone network was either twisted pair or open wire with transposition to guard against interference. Today, most of the twisted pairs in the world are outdoor landlines, owned and maintained by telephone companies, used for voice service.