Electrical cable

“ The energy of the currents and the quality of the wire also affect the speed. Until lately it was supposed that the wire acts only as a conductor of electricity, and that a long wire must produce a weaker effect than a short one, on account of the consequent attenuation of the electrical influence; but it is now known that, the cable being a reservoir as well as a conductor, its electrical supply is increased in proportion to its length. ”

“ Alterations in the Electrical Apparatus.—Since the manufacture of the cable in 1857, Professor Thomson had become impressed with the conviction that the electric conductivity of copper varied greatly with its degree of purity. As a result of the professor’s further investigations, the extra length of cable made for the coming expedition was subjected to systematic and searching tests for the purity and conductivity of the copper. Every hank of wire was tested, and all whose conducting power fell below a certain value rejected. ”

“ The weights of material used in this cable are: copper wire, 495 tons; gutta-percha, 315 tons; jute yarn, 575 tons; steel wire, 3,000 tons; compound and tar, 1,075 tons; total, 5,460 tons. The telegraph-ship Faraday, specially designed for cable-laying, accomplished the work without mishap.
Electrical science owes much to the Atlantic cables, in particular to the first of them. At the very beginning it banished the idea that electricity as it passes through metallic conductors has anything like its velocity through free space. ”

“ Robin looked at Letta, and wished that he were an electric cable!
“It happened to the Persian Gulf cable, quite recently,” continued Sam, addressing himself to Letta. “The cable between Kurrachee and Gwadur, a distance of 300 miles, suddenly failed one evening. Now, you must know that electrical science has advanced with such rapid strides of late, that we have the power to discover pretty nearly the exact position of a fault in a cable. ”

“ Insulated wires run from the test-box to instruments in the testing-room, so that the electrical condition of the whole151 cable is constantly under observation. During the whole time the cable is being laid its insulation is tested continuously, and at intervals of five minutes signals are sent from the shore end to the ship, so that a fault is instantly detected. The cable in its tank is eased out by a number of men, and mechanics are posted at the cable drums and brakes, while constant streams of water cool the cable and the bearings and surfaces of the brakes. ”

“ If, for instance, the contact resistance were equal to .001 of an ohm, and the wire were of such a size that .001 of an ohm were equal to the resistance of 20 feet of the cable, there would be an error of 20 feet in the location of the fault. For this reason all contact resistances throughout the loop from A to C must be extremely small. The battery is to be connected to the posts marked Ba., and the post marked Gr. is to be grounded. It will very frequently happen that the ground is to the cable sheath or some other conductor. ”

“ Canning, the engineer on duty, in setting men to work to cobble up the injury as well as time would permit, for the cable was going out at such a rate that the damaged portion would be paid overboard in less than twenty minutes, and former experience had shown us that to check either the speed of the ship or the cable would, in all probability, be attended by the most fatal results. Just before the lapping was finished, Professor Thomson reported that the electrical continuity of the wire had ceased, but that the insulation was still perfect. ”

“ Sir W. O’Shaughnessy, I believe, began it. Irishmen are frequently at the root of mischief! Anyhow, he, being Superintendent of Electric Telegraphs in India in 1839, hauled an insulated wire across the Hooghly at Calcutta, and produced what they call ‘electrical phenomena’ at the other side of the river. In 1840 Mr Wheatstone brought before the House of Commons the project of a cable from Dover to Calais. In 1842 Professor Morse of America laid a cable in New York harbour, and another across the canal at Washington. He also suggested the possibility of laying a cable across the Atlantic Ocean. ”

“ To remove cable armours, clamp the conductor firmly in the armour cutter and with a pair of cutting pliers back the armour off, one strip at a time, to the point of contact with the cutting edge of the tool. The vise for holding conduit takes all sizes. The conduit can be cut with an ordinary hacksaw. To protect the insulation against any possible injury while the wire is being drawn in, a soft metal bushing should be inserted in the end of the tube and secured permanently thereto by means of the bushing tool. ”

“ This may be done by joining the broken wire and a good wire at the distant end of the cable and measuring the resistance of the loop. To ensure close locations, this resistance should be over 100,000 ohms. Fair [524] locations can be made when the resistance is much lower and it is worth while to attempt it even if the resistance be as low as 10,000 ohms. The difficulty of determining the balance point increases as the resistance decreases. ”

“ Since practically all disturbances on lighting circuits are of low frequency, the series resistance can be used with good results; it aids the horn in extinguishing the arc, limits the size of the arc and prevents short circuits occurring during the period of discharge.
Fig. 2,382.—General Electric horn arrester for pole installation. Quite frequently series circuits are run underground in cables for some distance from the generating station. In order to protect the cables it is advisable to place horn arresters at the points where the cable joins the overhead wires. ”

“ The joints connecting these rubber covered wires and the cable conductors are spliced in the same manner as straight splices, the paper sleeves used being of sufficient diameter to be backed out of the way over the rubber insulation. When the splice is completed a brass shell threaded at one end to fit a female thread in the upper end of the brass base, is slipped over the end of the rubber covered wire and screwed into the base. ”

“ Then again, the electrical continuity—after all, the most important item—was perfect, and the electricians reported that the signals passing between the ships were eminently satisfactory. The door of the testing-room is almost always shut, and the electricians pursue their work undisturbed ”

“ In the case of continuous-current supply for incandescent electric lighting and motive power in small units, when the external conductors are laid down on the three-wire system, each main or branch cable in the street consists of a set of three conductors called the positive, middle and negative. ”

“ The insulated wires being drawn into them, any short circuit or heating of the wire cannot give rise to a fire, as it can only take place in the interior of a non-inflammable tube. A third system of electric light wiring is the safety concentric system, in which concentric conductors are used. The inner one, which is well insulated, consists of a copper-stranded cable. The outer may be a galvanized iron strand, a copper tape or braid, or a brass tube, and is therefore necessarily connected with the earth. ”

“ The attempt to insulate more than one conducting-wire or medium would not only have increased the chances of failure of all of them, but would have necessitated the adoption of a proportionably heavier and more cumbrous cable. The tensile power of the outer or wire covering of the cable, being very much less than that of the conductor within it, the latter is protected from any such strain as can possibly rupture it or endanger its insulation without an entire fracture of the cable. ”

“ The hard-drawn copper wire of high conductivity (invented by T. B. Doolittle in 1877) is invariably used; and wires weighing from 150 to 800 lb. per mile have been employed. It was necessary to carry the long-distance lines through underground cables in the approaches to large English cities, but owing to the inefficiency of cable wires as compared with aerial wires for speech transmission, the length of underground cable sections was kept down to a minimum. ”

“ In the case of the ungrounded sheath, the latter charges at once through all its length to the voltage of the high-potential wire; at some point, a wire within the cable is close enough to the sheath for an arc to strike across, and the trouble begins. All the wires in the cable are endangered if the cross be with a wire of the primary circuit of a high-tension transmission line. Any series arc-light circuit is a high-potential menace. Even a 450-volt trolley wire or feeder can burn a lead-covered cable entirely in two in a few seconds. ”

“ Who has not seen men in the streets dragging huge pipes of lead through the open manholes from big wooden spools? These are the cables you talk over. They have been papered and clothed—and tarred and feathered, maybe—and then encased in lead by a process that is so easy as to be laughable, and yet as ingenious as any one thing the wire miller does. Unrolling slowly from its spool, the heavy cable moves up to a machine built strong and four-legged from the floor. ”

“ In the main the causes which hinder a pulse committed to a cable are two: induction, and the electrostatic capacity of the wire, that is, the capacity of the wire to take up a charge of its own, just as if it were the metal of a Leyden jar.
Let us first consider induction. As a current takes its way through the copper core it induces in its surroundings a second and opposing current. For this the remedy is one too costly to be applied. Were a cable manufactured in a double line, as in the best telephonic circuits, induction, with its retarding and quenching effects, would be neutralized. ”

“ The electrician, however, not only used much higher power immediately he took the cable in hand—for working his specially devised relay and Morse electromagnetic recording-instrument in connection with his enormous induction-coils—but actually increased the power from time to time up to nearly 500 cells, till the five-foot coils yielded a current urged by a potential of something like 2,000 volts. ”

“ Cables employed between the generating and sub-stations of the Manhattan Elevated Railway, to distribute three-phase current at 11,000 volts, are of the three-conductor type, rubber insulated, lead covered, and laid in tile conduits. Each cable contains three No. 000 stranded conductors, and each conductor has its own insulation of rubber. Jute is laid on to give the group of conductors an outer circular form, and outside of the group a layer of insulation and then a lead sheath is placed. Outside diameter of this cable is nearly three inches, and the weight nine pounds per linear foot. ”

“ If single-phase or two-phase current is transmitted, each cable should contain the two conductors that go to make up a circuit. In work with alternating currents the use of only one conductor per cable should be avoided because of the loss of energy that results from the currents induced in the metallic sheath of such a cable. ”

“ He described 'high tension cables for 3,000 volts' having paper insulation impregnated 'with a rather thin liquid, oily and not too resinous mass' with cable joints bound with linen tape first boiled in oil so that it is completely saturated, with the wrapped joint inclosed in a lead sleeve, soldered to the lead sheath of the cable. Through a hole cut in the top of the sleeve or socket 'hot insulating mass is poured into the socket. ”

“ The natural increase in the number of aerial wires created a demand for better insulation and grouping in cables. Hundreds of miles of No. 12 iron wire were braided and dipped in suitable compound for this use. The annoyance from induction soon made a call for anti-induction cable. This want was supplied by a tin-foil cable so called, in which each conductor, after being insulated, was enclosed in a strip of this tin-foil. Cotton-covered wires to the extent of 50 or 100 were cabled together, and after being saturated with paraffine were placed in a lead pipe. ”

“ The design of the cable was a subject of experiment by Professor Morse and others. It was known that the conductor should be of copper, possessing a high conductivity for the electric current, and that its insulating jacket of gutta-percha should offer a great resistance to the leakage of the current. Moreover, experience had shown that the protecting sheath or armour of the core should be light and flexible as well as strong, in order to resist external violence and allow it to be lifted for repair. ”

“ In the most general practice each wire is insulated from all others by a wrapping of paper ribbon, which covering is only adequate when very dry. Cables formed of paper-insulated wires, therefore, are covered by a seamless, continuous lead sheath, no part of the paper insulation of the wires being exposed to the atmosphere during the cable's entire life in service. Telephone cables for certain uses are formed of wires insulated with such materials as soft rubber, gutta-percha, and cotton or jute saturated with mineral compounds. ”

“ In 1850 a cable twenty-five miles in length was laid from Dover to Calais, only to prove [Pg 40] worthless from faulty insulation and the lack of armour against dragging anchors and fretting rocks. In 1851 the experiment was repeated with success. The conductor now was not a single wire of copper, but four wires, wound spirally, so as to combine strength with flexibility; these were covered with gutta-percha and surrounded with tarred hemp. As a means of imparting additional strength, ten iron wires were wound round the hemp—a feature which has been copied in every subsequent cable (Fig. 58) . ”