ELECTRICAL SUBWAYS. Although there are no instances of subways for both pipes and wires in the United States, the practice of putting wires underground is rapidly growing. It is getting to be a comparatively common practice, even in small towns, for telephone companies to bury their wires; and though the work is often done cheaply and rather crudely, the wires are never theless removed from overhead and at the same time are so arranged that little subsequent dis turbance of the street is necessary. In a grow ing number of the larger cities electrical wires for light and power, together with the ordinary and the police and fire alarm telegraph lines, are also placed underground. Electrical subways con sist of a single pipe or tube, or a group of such, designed to protect underground wires from in jury due to settlement of the earth or the picks and shovels of workmen. At the same time these subways afford access to the wires through man holes placed at intervals, fo• repairs, connec tions, or the drawing in of new wires. Some of the underground systems have the conductors built in the tubes before the latter are laid, in which case new wires cannot be added without laying additional tubes, but provision is made for service connections without digging up the street. The most common materials used for the conduits or ducts are wrought iron, earthen ware. and wood, but sometimes cast iron, and again ordinary or more rarely bituminous ce ment concrete is used. In place of tubes troughs of cast iron, earthenware, o• wood, covered with the same material, are sometimes employed. The diameters of the various ducts are about three inches, and if one is not sufficient for the various wires to be buried any desired number may be placed in the same trench. Wrought iron pipes similar to gas pipes are commonly imbedded in concrete, and they may be laid in trenches lined with cement o• wood. Earthenware or terra-cotta ducts are generally surrounded with concrete, but the latter sometimes gives place to boards at the top. Each piece of conduit is eighteen inches long. If a group
of ducts is desired they are laid up one on another, breaking joints, much like brick work: or multiple ducts may be bought from the manufacturers. Concrete ducts are formed in place. Wooden ducts are in the form of square timber, with holes bored through the centre, or constructed more cheaply from boards.
Wood-fibre pipes are also used. (See PIPE.) In the Edison three-conductor tube system three copper rods are so wound, separately and col lectively, with rope, as to insulate them from wrought iron pipe, some twenty feet long, in which they are placed. These sections are filled with a fluid insulating compound before being shipped from the factory. Special insulated joints are made in the trenches, both for ordinary joints and for branch lines. Edison tubes of a simpler type but embodying the same principles are also made. The various classes of electrical wires and cables are generally insulated before being placed in subways. (See ELECTRIC LIGHT ING: ELECTRIC RAILWAYS; TELEGRA P II ; TELE PHONE. etc.) The wires and cables are drawn into the closed electrical subways through man holes at intervals of 200 to 300 feet by means of ropes o• rods. If ropes are used a steel wire with a round metal head is first pushed through, and by means of this a rope sufficiently large to haul the electric cable is drawn in. The rods are pushed through from manhole to manhole, one short rod after another being jointed onto the line. Ordinarily electrical subways need be placed hut feet or so underground.
Consult: Mason, "Street Subways for Large Towns," and discussion thereon. in Transactions of the Society of Engincens (London. 189i) article in Engineering Yews on "Street Subways for Pipes and Wires" (New York, March 15, 1900), describes all subways for both pipes and wires known to be in use up to early in 1900; also chapter on underground electrical construc tion in vol. ii. of Crocker, Electric Lighting (New York, 1901), and similar chapters in Hopkins, Telephone Lines and Their Properties (iii., new ed., 1901).