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 disadvan tages 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 uninsu lated, or bare, strips or rods of copper placed in tubes underground and held in position by insulators, or else the conduit itself is com posed 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 this system as many ducts as may be necessary are laid in a trench side by side and in layers, and manholes are built at intervals of 200, 300 or 400 feet to give access to the conduits and to afford means by which the cables may be drawn into the ducts. In a °drawing in* con duit system the ducts containing the "feeder° cables or cables for arc circuits are termed "trunk" ducts and are usually the lower tier or layer of ducts. The ducts carrying the distrib uting cables are termed "distributing ducts" and are placed at the top. So-called Thand-holes" are laid flush with the surface of the street every 40 or 50 feet to give access to the distrib uting ducts and cables for electric light and power service. One type of manhole is shown in Fig 12. This is a brick manhole; others are made of concrete. The hand pump shown is used to provide fresh air where gas is prevalent in the streets. The size of the manholes and number of ducts varies with the requirements of a given locality. Some manholes are from 4 to 5 feet square; others are 12 to 15 feet deep and 6 to 8 feet wide. The number of ducts in a conduit may range from 2 to 3 ducts to 200 or 300 ducts; the larger number usually being near the power-house or the telephone or tele graph headquarters. The manholes and hand holes are provided with double iron covers. Some of the covers are designed to make the manholes air- and water-tight; other covers are perforated to ventilate the conduits, to pre vent the accumulation of gas from adjacent gas mains, which occasionally causes explosions in the subways. The respective conductors in the
cables are joined together by twisting or by copper sleeves, in the.manholes; the conductors bemg separated from one another by insulating material. A lead sleeve is then placed over the joints and soldered to the main cable. A hot insulating substance, as wax or paraffin, is poured into the sleeve through a small hole in the sleeve, the holes being soldered thereafter. For telegraph and telephone distribution, pipes are run frorn the manholes into the vaults of an adjaccnt building, from which point the wires are led to the subscribers' offices in the block.
The cables are drawn into the ducts by means of a rope and windlass; they are usually too heavy to be drawn by hand. Electric mo tors carried on wagons are also used to draw irt the eables, the current for the motor being sup plied by a storage battery, an adjacent power wire or a portable gas engine. In order to get the-rope through the duct, a wire is sometimes placed in the duct as it is laid. More frequently, however, the ducts are rodded by means of a stiff steel wire, or by means of screw and socicet rods, similar to those used by chimney sweeps, one rod being screwed into its predecessor, which is then pushed along the duct until the distant manhole is reached, when a rope is at tached to one end of the rods and drawn through the duct.
For the ducts used in the drawing-in sys tem different material and varying lengths of pipe or tube are employed. At one time, wrought-iron pipe, 3 inches in diameter and 20 feet in length, joined together by thread coup lings and laid in hydraulic cement, was exten sively used in this country. About 5,000,000 feet of such pipe were laid and are still in service, but in recent years earthenware, terra-cotta or vitrified brick, stone and cement-lined pipe are mostly 'employed. The iron pipe and cement lined ducts are round, about three inches in di ameter. Many of the holes in the vitrified brick ducts are square, with an opening of about three inches. Fig. 13 illustrates a section of cement-lined pipe under construction. These tubes are of riveted sheet wrought iron and lined with five-eighths of an inch of pure ce ment The tubes are six to seven feet long and two to three inches in diameter, as required. A large quantity of these tubes are in use in this country and Great Britain.