A technical history of phantom circuits

Abstract
Phantom circuits, otherwise known as superimposed or derived circuits, were extra circuits obtained on a multipair telephone or telegraph cable, or open-wire pole route, by using both wires of one ordinary two-wire circuit, effectively in parallel, as one conductor of a phantom circuit. The use of suitable transformers and connections, together with special arrangements for balancing wire-to-wire and/or wire-to-earth impedances, enabled the phantom circuits to work with negligible interference to and from the ordinary (or ‘side’, or ‘physical’) circuits. The economic attractions were obvious. Invented in 1882, phantom working became practical around 1900 and was employed very extensively on open lines in the USA, and on cables in Britain, Europe and the USA, and probably elsewhere, in the subsequent few decades. The historical and technical development of the subject is outlined in this paper.

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