Illustrations of various types of underground cables are given in the accompanying figures. Fig. 4 represents a telephone cable; Fig. 5 a telegraph cable; Figs. 6 and 7 a one-conductor and two-conductor cable for high tension elec tric light and power circuits, respectively ; Fig. 8 a low tension, heavy current cable for electric light and power; Fig. 9 a three-con ductor electric power cable for 10,000-volt cir cuits. In this cable each conductor c is made up of a strand of 37 copper wires, each .082 inch in diameter. p is the oil-saturated paper or varnished cambric around the conductor, .17 inch thick. F is the jute filling. j is the paper jacket, also .17 inch thick. I. is the lead cover ing, .13 inch thick. The lead is usually alloyed with 2 or 3 per cent of tin. The outside diameter of this cable is 2.56 inches. The weight of each conductor is 4,000 pounds per mile; the weight of the lead covering is about 13 tons per mile.
Rubber and paper cables are now made to withstand pressures of 25,000 volts, and some miles of cable carrying current at this pressure are to-day in operation in underground con duits, but the ordinary operating pressure to-day is from say 2,000 to 11,000 volts for under ground cables.
Electric Underground Conduits.— The most obvious method of placing wires under ground•would be to provide a tunnel under the streets, in which not only the electrical con ductors but also the gas and water pipes of a city might be placed. This method is. how ever, so expensive that it has only been adopted in two or three places in the world, and then for only comparatively short distances in very crowded thoroghfares. For instance, there are several such tunnels in London, England, namely, the Holborn Street tunnel, about seven feet in height by 12 wide; the Queen Victoria Street subway and the Victoria Embankment tunnel, seven feet by nine feet. The total length of these London tunnels is about six miles and they cost approximately $140,000 per mile, in cluding ventilators, side passages and entrances.
In some of these tunnels, water and gas pipes, pneumatic tubes and telephone, telegraph and electric-light wires have been placed. In Paris at one time some of the sewers were utilized for the same purpose, but this plan was not greatly favored and has .iot been followed else where. Tunnels for electrical conductors were also built in-Detroit, Mich., the longest of which is about 232 feet in length. It is six feet six inches by three feet six inches in the cross-sec tion.
Solid Conduits.— Another plan which has been utilized for this purpose is one in which the conductors are well insulated and laid di rectly in the earth ; or in which the conductors are laid in notches in a tube or duct, by which means they are kept apart. The tube is then
filled with an insulating compound, which, when it hardens, holds the conductors securely in po sition. This is termed a °solid" conduit. One of the earliest forms of solid conduit was that used by Morse, between Washington and Balti more. This consisted of five wires insulated with cotton and placed within a lead tube which was laid directly in the earth. In different parts of Europe, in the middle of the last century and afterward, wires were laid directly in the earth without other covering than the insulating material around them, which was usually a bitu men compound or gutta-percha. Insulation laid in this way is not long lived. One of the first solid conduits used in this country for electric lighting was one in which a lead-covered cable is laid directly in a wooden trough, the cable being uncoiled directly from a cart reel, the box being then filled with an insulating compound. To protect the cable from injury, a thick plank was placed over the box.
In many European cities solid conduits are placed under the sidewalks. The cables c are laid on a bed of sand, s, as indicated in Fig. 10. A galvanized iron wire netting, x, is placed over the sand, separating it from a bed of concrete, x, upon which the asphalt, A, of the sidewalk is laid. The object in using the wire netting is to warn workmen of the presence of the cables.
Edison Solid or Iron Tube Conduit.— This is the conduit adopted by Edison for the dis tribution of electric current by the three-wire system, for light and power in cities. It con sists of an iron tube about 20 feet in length, into which the three conductors, usually copper rods, separated from one another by hemp or jute cords, are inserted. An insulating com pound is then forced, under heavy pressure, into the tube at a temperature of about 300° F. The copper rods project about two inches at each end out of the tube. The tubes are laid end to end in the earth, when the conductors in one tube are connected to those in the next by a flexible copper strand. A split iron box is then jointed and clamped over the ends of the tube and the box is then filled with an insulat ing compound through an opening, which is then closed by a screw plug. In this system no manholes are employed, but instead, at suitable distances, water-tight junction boxes are used, into which the conductors are led, as outlined in Fig. 11. This is really a switch-box, by means of which the current from the conductors is distributed to the °mains° or "service" conductors. These boxes are also utilized to break up the mains into shorter sec tions; to open the circuits for testing and other purposes.