there has not been a pole supporting telegraph, telephone, electric light or trolley wire in any part of Manhattan borough, such wires all being placed in cables in conduits under the surface of the streets.

Electric Underground Cables. The type of underground cable used for telephony, telegraphy and electric light and power purposes varies greatly. For example, the conductors used in telephony have a diameter of .040 inch; those for telegraph purposes about .080 inch; those for electric light and power range from one-quarter of an inch to one inch and over in diameter. The smaller electric power wires are employed in high potential and comparatively light current work; the larger wires in low tension and heavy current work. It is thus feasible to place about 400 telephone conductors, or 100 telegraph conductors, in one cable in a three-inch underground duct or pipe, while it is only practicable to place two, three, or, at most, five electric light or power conductors in a similar duct. The insulating material used for telephone conductors is usually a wrapping of tissue paper in narrow strips, laid on spirally over each conductor. The insulating material of telegraph underground cables is usually a rubber compound or strips of paper saturated with oils, the thickness of the wall of which is about .038 inch. The insulating material of electric light and power cables is usually a rubber compound, oil paper or varnished cambric, which is from one-eighth of an inch to nearly half an inch thick, depending on the electric pressure to be withstood, which, in the case of low potential circuits, is about 220 to 600 volts, and in the case of high potential circuits may range from 1,000 to 30,000 volts. Gutta-percha, which has been employed for the insulation of long submarine cables, is not used for underground cables, owing chiefly to its low softening point under heat, 120° F., which temperature is not infrequently encountered in subways in cities.

Cables designed for underground work are encased in a lead envelope to protect the insulating material from water, moisture and the effects of gases, acids, etc., in the underground conduits. For crossing rivers such cables are also armored with iron wires in addition to the lead covering, as a mechanical protection.

The term cable includes the conductor ("core"), the insulating material, the lead covering and the armor when the latter is employed. Copper is practically the only metal used for the conductors of electric cables. Aluminum is not used because of its bulk for a given conductivity, which bulk is about 1.6 greater than copper. The increased amount of insulating material and lead covering, as well as space in the conduits, that would be required in the case of aluminum for a given conductivity would be virtually prohibitive of its use for underground cables.

The copper wire used in cables is drawn to the required size in the wire factory. If the wire is to be insulated with a rubber compound it is "tinned" to prevent any chemical action between the sulphur used in the rubber compound and the copper. When the covering is paper, linen or fibre the wire is not tinned. The tinning process consists in passing the wire through a vat of molten tin. For electric light and power cables, when the conductors do not

exceed 204 inch diameter, they are usually solid, or of one wire; above that size they are generally stranded to obtain flexibility. The wires are stranded in a stranding machine in one process, the wires being wound on reels, which are held on suitable spindles on the frame of the machine. A single wire is held in the centre of the frame and is slowly drawn through a guide. The wires for the first layer are wound spirally around the central wire; the wires for the second layer are held on another frame and are laid over the first layer in an opposite. direction, and so on for the additional layers required. The strand is wound upon a drum and is then ready for the insulating process.

Rubber Insulation. The rubber used in the insulating material for cables is pure Pará rubber. After the rubber has undergone treatment by washing and kneading to remove the impurities which it always contains in its crude state, it is then mixed, by suitable machinery, with the ingredients that go to make up the compound, such as litharge, whiting, blue lead and sulphur. The compound is then ready for placing over the wire. There are two general methods by which this is done, termed, respectively, the seam and seamless methods. In the seam process the rubber compound is calendered into a sheet of any required thickness, which is then cut into long strips. These strips are then passed between two grooved rollers having sharp cutting edges. The wire to be covered also passes in the centre of the grooves of these rollers, and as it does so the rubber strips are pressed closely around it, the knife edges of the rollers cutting off the surplus rubber strip. The wire thus insulated is fre

H

FIG. 1.- Rubber Covering Machine.

quently wrapped spirally with a tape, after which it is placed in a vulcanizing oven and vulcanized. In the seamless method the compound is placed in a plastic condition around the conduits by pressure, while passing through a die. The conductor, c, Fig. 1, is drawn through a metal chamber or box, B, which contains the plastic compound. A worm gearing, w, within the chamber, pushes the compound toward the opening or die, d, in the end of the chamber. The compound is fed into B at the aperture A. The chamber is kept at a desired temperature by a hot water or steam jacket. After leaving the chamber the insulated wire is drawn slowly along a table, through powdered talc to prevent sticking, to a drum, on which it is then taken to the vulcanizing box or receptacle, unless it is first to be taped. The taping process is somewhat analogous to that of stranding the wire. A vertical taping machine is shown in Fig. 2, in which the insulated wire wis seen coming through the floor to the guides c c, in each of which there is a slot through which tape from the small reels R R passes to and around the wire. The wheels on which the reels R R are carried revolve in opposite directions, this action laying the tapes on

and power and telegraph service are made up of reversed layers of strips of manila paper to a desired thickness by means of a paper-covering machine such as is indicated in Fig. 3. In this figure wis the wire moving in the direction of the arrow. By suitable motive power the reels R carrying the paper strips are revolved in opposite directions around the wire.

FIG. 8.- Heavy Current Cable.

When thus covered the conductor is wound on a reel and placed in an oven until all moisture is driven out of the paper. The reel, with the insulated conductor, is then immersed in a vat of boiling oil for several hours until the paper is thoroughly impregnated with the oil.

Varnished cambric insulation consists of strips of varnished and oiled linen cambric, which are placed over the conductor in as many layers as may be desired, varnish being applied between the layers.

When insulated the conductors are ready for their lead covering, if to be used as single conductors; or if to be employed in cables, they are now ready for cabling. In the latter case the number of conductors in a cable will vary with the purpose for which the cable is designed. Telephone cables for underground use may consist of as many as 400 conductors, which are first twisted in pairs and are then cabled by a cabling machine virtually similar to a stranding machine. For telegraph uses single conductors, to the number desired, are laid up spirally in

FIG. 9. A Jacketed 7,000 to 10,000 Volt Paper Cable. the cable. For long underground telegraph working a special type of cable has been devised to avoid the effects of static induction between conductors. This is termed a screened cable from the fact that each conductor after being insulated is covered with a thin copper ribbon laid on spirally and overlapping. The conductors thus screened are cabled in practically

the usual way. The copper ribbon over each insulated conductor is grounded by connecting it with the lead covering of the completed cable. The static lines of force set up by the telegraph currents in the conductor expend their energy in setting up induced currents in the copper ribbon and thus the conductors proper are protected or screened from the effects of parallel static induction.

For electric light and power purposes, espe cially for high potential circuits, three conductors in one cover are now generally used. These conductors are laid up spirally and taped, the spaces between the conductors being filled with jute rope, in the act of cabling. In other instances the three conductors are bunched and a "jacket" of paper is laid over them. This is termed a jacketed cable or "split" insulation. The cable thus laid up is then taped after which it is ready for the lead covering.

The process of lead-covering cables is as follows: The cable is drawn through a die in a die-block, and, as it passes through this die, hot lead in a semi-plastic state is pressed in a uniform thickness around the insulating material by pressure from a hydraulic ram. The pressure exerted on the end of this ram sometimes amounts to 500 tons.

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 electric light and power circuits, respectively; Fig. 8 a low tension, heavy current cable for electric light and power; Fig. 9 a three-conductor electric power cable for 10,000-volt circuits. 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. L is the lead covering, .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 conduits, but the ordinary operating pressure to-day is from say 2,000 to 11,000 volts for underground cables.

Electric Underground Conduits.- The most obvious method of placing wires underground would be to provide a tunnel under the streets, in which not only the electrical conductors but also the gas and water pipes of a city might be placed. This method is, however, 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, including 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 not been followed elsewhere. 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-section.

Solid Conduits.- Another plan which has been utilized for this purpose is one in which the conductors are well insulated and laid directly 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 position. This is termed a "solid" conduit. One of the earliest forms of solid conduit was that used by Morse, between Washington and Baltimore. 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 bitumen 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

FIG. 10.- Sidewalk Conduit.

laid on a bed of sand, s, as indicated in Fig. 10. A galvanized iron wire netting, K, is placed over the sand, separating it from a bed of concrete, N, 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 distribution of electric current by the three-wire system, for light and power in cities. It consists of an iron tube about 20 feet in length, into which the three conductors, usually copper

FIG. 11. Edison Junction Box.

rods, separated from one another by hemp or jute cords, are inserted. An insulating compound 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 insulating 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 "feeder" conductors is distributed to the "mains" or "service" conductors. These boxes are also utilized to break up the mains into shorter sections; to open the circuits for testing and other

purposes.

The disadvantage of "solid" conduits is that in case of defects in the cables there is no means of repairing them short of tearing up the streets. Neither is it convenient to add to or take from or to increase or diminish the size of conductors used in the "solid" system. These disadvantages do not exist in the case of what is termed the "drawing in" conduit system, to be described presently.

Bare-wire Conduits.- Still another plan utilized in some parts of Europe, and known as the "bare wire" conduit, consists of uninsulated, or bare, strips or rods of copper placed in tubes underground and held in position by insulators, or else the conduit itself is composed of an insulating material and is protected from moisture. This plan is not in extensive

use.

Drawing-in Conduit.- The method which is now most generally employed in this country is that known as the "drawing-in" conduit. In