CABLE, Strictly speaking, a com bination of two or more separately insulated electric conductors with a protective covering or armor. Popular usage sanctions the extension of the term electric cable to simple insulated and armored wires and to ropes of twisted wires without either insulation or armor. for the present the liinited definition first given, an electric cable may be described as consisting structurally of. first, the conducting wires or core; second, the insulating material separating the several wires; and third, the protective cov ering or armor. Cables may le aerial, sub marine, or underground, depending on their position: classed according to their uses they are telegraph, telephone, electric-light. and power cables, and as to the arrangement of their conductors they are straightway or twisted. Cables for different purposes differ somewhat in the details of their construction, but their general construction is substantially similar. and the following description of underground cables for electric lighting will answer for caldes for other purposes, with sueh exceptions as will be noted farther on. Electric-light cables for use underground are of two classes, according as the insulation is or is not moisture-proof. Li the first (lass the insulation is rubber or bitumen and the lead covering is for protection from mechani cal injuries only. The second class is insulated with jute, hemp, or paper impregnated with oil. wax, or resinous compound, and the lead cover ing for this cable is absolutely necessary on account of the ligroscopic nature of the insula tion. The manufacture of a cable of the first class may be described briefly as follows: To insulate the conductor it is first wrapped around with one or more layers of pure rubber tape put on spirally. the direction of the spiral being reversed for each successive layer. On top of this rubber compound is applied in two or more separate coatings. each coat being put on by pressing the partially formed cable with two strips of rubber compound. one above and one below it, between a pair of rollers which fold each strip half around the cable and press the edges of the two strips together so as to make a good joint along each side. When a sufficient number of layers of rubber compound have been put on to give the requisite thickness, the core is tightly bound in a spiral wrapping of pre pared rubber tape and then vulcanized. After this the cable is tested to determine the efficiency of its insulation. If this test is satisfactory the cable is taken to the taping and braiding machine. where the external covering of tapes and braid ing is put on The next step is to armor the cable with lead. This may be done by drawing the cable into a lead tube, which is then drawn through a die and made to fit the core tightly: or the last cover may be put on in a hydraulic press, the hot lead being forced out through an annular (lie around the cable. The accompanying sketch shows the make-up of an electric-light underground cable before it is armored. Of
cables of the second class the Siemens cable and the paper cable are representative examples. In the Siemens cable the conductor is wrapped with jute and impregnated with a special bituminous eenupound mixed with heavy oil, and is then cov ered with lead. Paper cable consists of paper wound on in strips spirally over the conductor, and as each strip is applied the whole is passed through a die which presses it into a compact mass. The core is then dried at a of 250° F.. to expel the moisture from thepaudr, and immersed in a bath of speciafly prepared com pound, from which it passes directly to the lead covering press.
The standard type of eable for telephone work consists of 400 insulated wires twisted in pairs with about three-inch lay: and the pairs are cabled in reverse layers, forming a cable about two Melees in diameter. The 200-pair cable is used for main routes. but 100-pair, 50-pair. and smaller cables are used for distribution. The in sulation consists of dry paper wound loosely on the wire, and the whole is armored with lead pipe. Submarine cables for telegraph and tele phone lines are much like the underground cables of the first class for electric lighting, but are commonly armored with strands of iron wire. (See TELEGRAPHY. SUBMARINE. ) Aerial cables for long-distance power transmission are commonly neither insulated nor armored. For example, the aluminium cable for the 1S1-mile transmis sion line of the Bay Counties Power Company. in California. is seven-eighths of an inch in outside diameter and consists of :3"4" aluminium wires twisted into a rope without insulation or armor. These cables carry a current of 40,000 volts. ..Wrial cables for local distribution are usually insulated. This insulation is usually in two parts: one of insulating material impervious to moisture, placed next to the wire, and the other of some substance which resists abrasion or other mechanical injury. In the most expensive grades of wire more than two coatings are employed. Various special cables are employed. For ex ample. there is in use in New York City a three conductor cable for transmitting three-phase lighting current from the main stations to sub stations. This cable may be broadly described as consisting of three separate insulated cables of 37 wires. twisted together with the spaces filled with jute and the whole first insulated and then armored with lead pipe. Concentric cables are another special form. They consist first of one wire core of twisted wires, second a thick layer of insulation, third a layer of spirally wound wire or wire strands, fourth a layer of insulation. and fifth a protective coating or armor. See TELEGRAPHY, SCIMARINE. Consult the sections relating to electric cable construction to be found in Crocker. Electric Lighting, Vol. 1]. (New York. 19111) ; Foster, Electrical Engineer's Poeket-Book (Ne• York, 1901) ; Houston. .1 Diet io na ry of Electrical ll'o•ds, Terms, and Phrases (New York, 1894),