Telegraph

Conductors in cables designed for telegraph service are of soft-drawn copper varying in diameter from 0.036 in. to 0.102 inch. Ordinarily, cable conductors are separated and insulated by means of paper wrapped spirally about them, although cables having rubber-insulated conductors are often used for short dis tances. The insulated conductors are arranged in pairs, which consist of two conductors twisted together or in quads each con sisting of two pairs twisted together. These pairs or quads are arranged compactly in layers. Paper-insulated conductors are always enclosed in a lead sheaths in. in thickness; rubber-covered conductors may be enclosed either in a lead sheath or in a cov ering of heavy braid. The number of conductors provided in telegraph cables varies, the largest now in common use being 404 pairs of paper-insulated conductors 0.036 in. in diameter. The outside diameter of this cable is in. and the weight is 7* lb. per foot.

Overhead Lines.

Poles used to support wires and cables are usually of wood, although steel and concrete poles have been used. They are of such height as to provide a suitable clearance from the ground for the wires and cables. The size of poles required and the distance they are placed apart depends upon the geographical location of the line and upon the number of wires and cables car ried. Lines situated in sections of the country where ice or snow is apt to collect upon the wires require the use of larger poles placed closer together than lines in portions of the country where ice never or rarely forms. In more favourable climatic locations and if only a few wires are supported poles are often placed 175 ft. to 200 ft. apart. It is not considered in general good practice to. place more than sixty wires on a pole line, although eighty or even a hundred wires are sometimes carried for short distances where it is not too expensive to place the poles close together.

It is common practice to impregnate wood poles used in tele graph lines with some form of preservative in order to retard decay. Until recently creosote was used almost exclusively as a preservative for poles, but certain insoluble arsenic compounds are now employed extensively. (See TIMBER PRESERVATION.) Bare wires are supported on insulators usually made of glass and held in position by wire ties. These glass insulators are mounted on either wood or steel pins which are set vertically in crossarms of wood.

Crossarms are normally Io ft. in length and accommodate ten pins spaced approximately 12 in. apart. For lines carrying a small number of wires 6 ft. crossarms carrying six pins are often used. These crossarms are securely fastened to the pole, the top one approximately 12 in. below the top of the pole and additional arms each 2 ft. below the next higher arm. Cables when placed on poles are supported by suspension strands, consisting of seven high-strength galvanized steel or bronze wires twisted together.

The weight of the cable to be supported determines the strength of suspension strand used, the heavier cables requiring strands having a maximum strength exceeding 18,000 pounds.

Cables are hung from suspension strands by means of metal rings clamped on the strands from I to 2 ft. apart. The sus pension strands are fastened to the poles by means of strong metal clamps bolted to the poles. At corners, along curves, and at the ends of lines, it is often desirable to strengthen the line by the use of guys of steel strand similar to that used for supporting cables. These guys are fastened at the upper ends to the poles and at the lower ends to logs or other substantial objects buried in the earth. The purpose of these guys is to prevent the poles from being pulled over or broken by the constant pull of the wires.

Underground Construction.

Where cables are placed un derground some form of conduit, with independent duct space for each cable, is usually employed to permit the cables to be readily installed and removed. The ducts are usually of hollow tile, creo soted wood or fibre. Iron pipe is used in special situations. All ducts are 3 in., or slightly more, in diameter. The conduit is buried to a depth of at least 2 ft. in the ground. As the length of cable which can be pulled through a duct is limited, splicing chambers or manholes are provided at intervals of 500 ft. to 600 feet. Manholes are normally at least 3 ft. wide, 5 ft. long and 41 ft. deep, although in many cases considerably larger sizes are required. They are usually constructed of brick or concrete with a removable cover.

Cable is pulled into conduits one length at a time and then the ends are spliced together. In splicing paper-insulated cables a foot or more of the sheath is removed from the end of each of the cables to be spliced, and a few inches of the insulation is removed from the conductors. The conductors are then joined by twisting, soldering or other means, and the joints are insulated by being covered with small paper or impregnated cotton tubes. After all conductors are joined the splice is wrapped with strips of muslin and a lead sleeve is slipped over it. This sleeve is sealed at each end to the cable sheaths by means of wiped solder joints.

At points where overhead wires connect to paper insulated cables, it is usually necessary to provide electrical protection to prevent high voltages, due to lightning or to accidental contact with power wires, from puncturing the insulation of the cables. Accordingly arresters similar to those used in offices are com monly placed on poles, at which junctions between wires and cables occur. Where large currents are apt to result from contact with power wires, fuses are also used to prevent the cable con ductors from being burned out. Protector equipment is usually housed in boxes mounted on the junction poles.