Rubber Insulation.— The rubber used in the insulating material for cables is pure Para rubber. After the rubber has undergone treat ment by washing and kneading to remove the impurities which it always contains in its crude state, it is then mixed, by suitable machinery, ivith 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, re spectively, the seam and seamless methods. In the seam process the rubber compound is calen dered into a sheet of any required thiclaiess, 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 lcnife edges of the rollers cutting off the surplus rubber strip. The wire thus insulated is f re quently wrapped spirally with a tape, after which it is placed in a vulcanizing oven and vulcanized. In the seamless method the com pound 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, s, which con tains 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 instilated 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 re ceptacle, unless it is first to be taped. The taping process is somewhat analogous to that of stranding the wire. A vertical taping ma chine is shown in Fig. 2, in which the insulated wire w is 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 oppo-, site directions, this action laying the tapes on the wire in reverse spirals. The wire thus taped passes to the "take up" drum T, thence to the reel D. In the case of rubber-covered wires the next proceeding is to immerse them in a water tank for 12 or 24 hours, after which they chief ingredient in the compound that brings about these results. The compound usually enters the oven a yellowish compound and comes out a dark-blue color. This color may be varied by using different ingredients in the are electrically tested for defects in the insula tion that may be due to air-holes, foreign sub stances in the insulation or any other cause.
The vulcanizing process consists in placing the insulated wire in an oven, where it is kept compound, and in some cables certain of the conductors are colored by this means to act as amarkers," or distinguishing wires for testing purposes.
Paper conductors intended for telephone work are covered very loosely with two layers of dry, soft paper, laid on spirally, in practically the manner in which tape is placed over the rubber insulated wire. This type of insulation is found to be the most at a temperature of 250° to F. until the rubber compound is brought to a desired degree of hardness and tenacity, the proper time for which is a matter of experiment and varies with different compounds. Sulphur is the
satisfactory yet devised for telephone cables, its capacity being quite low, about .080 microfarad per mile of conductor. The insulation resistance of each conductor is about 500,000,000 ohms per mile. The wires thus insulated are twisted in pairs with a lay of about three inches, the pairs being laid up in reversed layers and built up into cables of 50, 100 and 200 pairs, after which they are lead covered as a protection against moisture. Paper cables for electric light and power and telegraph service are made up i) t 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 w is the wire moving in the direc tion of the arrow. By suitable motive power the reels a carrying the paper strips are revolved in opposite directions around the wire.
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, vyhich 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 con ductors; 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 con sist 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 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. Thecon ductors 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 i for high potential circuits, three conduc tors n 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 in stances the three conductors are bunched and a itjacketn 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 pres sure exerted on the end of this ram sometimes amounts to 500 tons.