Identifier: belltelephonemag08amerrich (find matches)
Title: Bell telephone magazine
Year: 1922 (1920s)
Authors: American Telephone and Telegraph Company American Telephone and Telegraph Company. Information Dept
Subjects: Telephone
Publisher: (New York, American Telephone and Telegraph Co., etc.)
Contributing Library: Prelinger Library
Digitizing Sponsor: Internet Archive

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Chiefly in the large cities, there are places where all the ducts in the existing underground conduit systems of the telephone company are becoming occupied with cables. To provide more wires for the constantly growing telephone plant, two courses are open; either to install more underground ducts to accommodate additional cables or to replace some of the cables which now occupy ducts in the existing conduit system with cables containing more wires. In many of these situations, on account of the extent to which the space beneath the streets is occupied by rapid transit tunnels, sewers, water and gas pipes, electric light and power cables, and other subterranean works, it would be possible to provide additional telephone ducts only with great difficulty and at large cost.

Hence the relief afforded through the ability to substitute 1800-pair cable for 1200-pair cable, or smaller, either in new work or in the existing plant, is a matter of importance. Furthermore, since every underground cable occupies a duct and must have a metallic sheath, the expense of a pair of underground wires becomes less in proportion as the cost of duct and sheath is divided among a larger number of pairs of wires in the cable. So where the use of 1800-pair cable, rather than 1200-

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pair, is practicable, from a transmission standpoint, this will assist in keeping the cost of the telephone plant from mounting as rapidly as would otherwise be the case.

1800-pair cable will also affect plant engineering practices, by changing the economic relation between the costs of material and of labor. For example, where growth is rapid, there will be cases in which greater economy and better service will result from providing relief by means of new 1800-pair cable than by making a greater use of existing facilities through rearrangements and changes.

Lest it might be assumed that the development of 1800-pair cable consisted merely in substituting 26-gauge wire for the 24-gauge wire used in 1200-pair cable, it seems worth while to point out that the development required the invention of a wholly new method of forming the cable core. The type of assemblage previously used, consisted of layers of twisted pairs, laid up in the form of a helix, adjacent layers being stranded in opposite directions. This type of assemblage did not prove suitable for making 1818-pair cable and it became necessary to devise a core which virtually consists of an assemblage of 18 component cables or "units," each containing 101 pairs. These constituent cables are grouped together as shown in Figure, and this peculiar type of core construction has been given the name "multiple unit." The multiple unit core structure will undoubtedly find application in other types of cable. Each wire in this 1800-pair cable is insulated by means of a wrapping of paper tape. As in the 1200-pair cable, the paper tape is about one-quarter of an inch wide, but in the 1800-pair cable, the thickness has been reduced considerably.

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