Phantom Circuits.— It is possible to make a third circuit out of two metallic circuits by connecting them together, with suitable appa ratus, in a peculiar way. (Fig. 47). The third circuit is termed a aphantomn circuit. Con versations can go on over a phantom circuit without disturbing, or being disturbed by, simul taneous conversations over the two metallic cir cuits from which the phantom circuit is formed. The operation of the phantom circuit depends upon the principle that if two currents of exactly the same strength, and similar in all respects as to form, are transmitted simultaneously both sides of the upper circuit in parallel, as described above, returning in a similar manner over both sides of the lower circuit. So far as the phantom is concerned, the two wires of the upper and lower circuits, respectively, work together each as one side of the phantom circuit. While the phantom cir cuit was first proposed about 1884. it was not until important work had been done in de veloping highly perfected and balanced repeat ing coils, and in working out with great pre cision the balancing of the circuits themselvesi that the phantom principle came into success ful commercial use.
Aerial and Underground Cables. Early In 1881, the first experimental underground cables were laid alongside a rail road track at Attleboro, Mass. The best insula tion known at that time, rubber or guttapercha, was found to be unsuited for telephone use. Conversation conducted through cables -using that kind of insulation was muffled, and the overhearing from one circuit to another was troublesome. In 1882 several cables were laid at Boston, Mass., the longest of which was about 1,500 feet. It was found that when these cables were used in connection with lineS reaching to the suburbs, the voice became so indistinct that, unless the difficulty were re,. moved, the connection with points outside the city would have been almost, if not quite, use less to those whose wires were underground. Type after type of cable was installed only to be withdrawn in a few years and replaced by something better.
The use of rubber insulated cable was fol lowed by the development of a cable in which the individual wires were covered with cotton insulation and drawn into a pipe which was then filled with oil. This oil-filled cable carried the telephone business through half a dozen years but by no means•was it the final type.
This was followed by the use of cotton cov ered wires having the cotton impregnated with a moisture-proof compound, the insulated wires being drawn into lead pipes, each section about 20 feet long. When the cable was laid the sections were connected by means of plumbers' joints. The next development resulted in the
use of cable• wires insulated with dry cotton, it having been found that cotton baked dry, so as to expel all moisture, was a good insulator if placed in a lead pipe and promptly sealed. This was an important development as cotton had formerly been considered a bad insulator. Moulding hot lead in a continuous sheath around the cable core of twisted wires was found to be an advantage as it meant a tight covering with fewer sleeves and splices and ultimately did away, with the use of oil or moisture-proof compounds. By 1887 the in troduction of twisted pair construction for con ductors in cable began. This type of construc tion carried out the application of the trans posed metallic circuit principle to cables. It obviated interference from overhearing (knoWn in the art as "crosstalko) between circuits in the same cable. This improvement meant the abandonment of the entire existing cable plant and the introduction of the new type, without which the telephone system, as it is known to day, would have been an impossibility.
Insulated Paper More development work, extending over several years, constantly aiming to reduce the specific in ductive capacity of the insulating material, led to experiments with paper twine. This proved too flimsy and methods were devised for put ting paper on the wire flat, crumpling it around the wire instead of winding it tightly. This resulted in the production of the best type of insulating material known for telephone cables, namely, dry paper hermetically sealed in a con tinuous lead sheath. In 1890, adry-core') cable, the first of the modern type, was laid in Phil adelphia. The conductors, wound with loose paper, were really cushioned largely with air, the best insulator known. This was the best talking cable that had ever been made up to that time. The first dry-core paper cables con tained 50 pairs of wires No. 18, B. & S. G. in size (0.0403 inches in diameter). By 1891, the use of wires No. 19, B. & S. G. in size (0.0359 inches in diameter) was settled upon. Subse quent progress in cables employing this size of wire has been largely in the direction of in creasing the number of pairs of wires that can be placed in a full-sized sheath. This has been accomplished chiefly through improvements in the insulating paper and in the mettods of ap plying the paper to the wires. The progress made is shown in the following table which gives, for cables of No. 19 gauge wires, about 214 inches in outside diameter, the date when a cable of the given number of pairs was made available for commercial use: Number of pairs of