English: The difficulties encountered in working underground cable circuits were exhaustively investigated by the British Post Office in the first decade of this century, and it was proved that the inability to obtain phantom circuits was due primarily to want of balance between the electrostatic capacity of conductors in respect to (a) other conductors and (b) to earth. As a result of the earlier investigations the method of laying up the conductors in pairs to form a complete cable was radically changed. A type of cable known as the “quadruple pair” was introduced. In this type the conductors are lapped with insulating paper twinned together in pairs, and are arranged in “cores” each containing four twisted pairs laid up together around a centre, usually of yarn, forming a “quadruple pair” core. The cores are laid up together to the number required and sheathed with lead. This type of cable was a great improvement on the earlier “twin” cables, and permitted the formation of a superposed “phantom” circuit on two physical circuits. Diagonal pairs in the same core are selected for superposing.

In a later type of cable known as the “multiple twin cable” the centre of yarn is dispensed with, and the cable consists of a number of 4-wire cores made up of two 2-wire cores twinned together. The Manufacture of this type of cable has been greatly improved in recent years, and cables are now produced with very small out-of-balance capacities between wire and wire, and between wire and earth. It is still, however, necessary to balance the cables after laying by a systematic method of jointing contiguous lengths, whereby conductors are selected and jointed in such a manner as to secure maximum uniformity of characteristics.

A method of loading the phantom circuit in telephone cables was invented by G. A. Campbell and T. Shaw in the United States and patented in Great Britain in 1911. This method was applied to a cable laid between London and Birmingham in 1914 and extended to Liverpool in 1916.

The phantom circuit is obtained by means of specially wound transformers joined across the ends of the physical circuits. The cores of these transformers consist of a ring made up of very fine soft iron wires. The figure illustrates the method of connecting.