Wires that are to convey electricity under ground, or through water, must be defended from injury and insulated to prevent the lateral escape of the electricity. For that purpose the cable that is laid at the bottom of the Atlantic, from near Valentia in Ireland to Trinity Bay in Newfoundland, is formed of seven fine copper wires which convey the electricity, bound together by a coating of gutta percha, over which there are layers of cloth dipped in pitch, and then the whole is covered by steel wires twisted together in strands and twined round in long close spirals, forming a cord or cable not more than an inch and a quarter in diameter, and 2100 miles long. The use of the gutta percha is to insulate the wires; the other coatings are merely for protection.
The Voltaic battery which generates the electricity consists of 40 cells, the plates of which are alternately of zinc and platinized silver, each about nine inches square, the exciting fluid being dilute sulphuric acid. Although the force developed by this battery is so great that a piece of iron three inches long and three eighths of an inch in diameter placed in contact with the poles may be consumed in a few minutes, it is absolutely incapable of sending a current of electricity through wires 2500 miles long, on account of their resistance, without the aid of Dr. Faraday’s inductive action. It is only the primary agent for inducing a current of sufficient strength.
To accomplish that, many thousand yards of fine copper wire coated with silk are wound round a hollow soft iron cylinder; the whole is then coated by gutta percha, and the end of the wire is joined to the wires in the cable so as to form a continuous line from Valentia to Newfoundland. A second copper wire, shorter but thicker than the preceding, and also insulated by a coating of silk, is wound round the cylinder above the gutta percha: when the ends of this thick wire are brought into contact with the poles of the battery, currents of electricity flow through it, between pole and pole, and in their passage temporarily convert the hollow iron cylinder into a powerful electro-magnet, which by its reaction induces a current of electricity in the fine wire of sufficient power to cross the Atlantic. The efficiency of the electric telegraph depends upon the power we possess of breaking and renewing the current at pleasure, since by that means distinct and successive signals are made from station to station. In the Atlantic cable positive and negative electricity are transmitted alternately; the electricity is sent to America from alternate poles, and the current returns again through the water, which completes the circuit.
The passage of electricity through a cable or telegraphic wire in air is sensibly instantaneous; that through a cable, whether extended in water or under ground, requires time on account of lateral induction through the gutta percha; for the electricity, in passing through the wires, induces the opposite electricity on the surface of the water or moist earth in contact with the cable, and in that respect it is precisely like a Leyden jar, the gutta percha representing the glass. As the power of induction is proportional to the tension of the electricity, and as the tension is continually diminished by the resistance of the wires, the induction is continually diminished and requires a longer time. Electricity took two seconds to pass through a cable 768 miles long, laid under ground from London to Manchester, and back again twice; while in air it was all but instantaneous, because the inductive capacity of air is very much less than that of water or moist earth. In the experiment with the cable under ground it took two-thirds of a second to overcome the resistance of the wires, and then the velocity of the electricity was 1000 miles in a second, and it was the same whatever the intensity of the electricity.
It has already been mentioned that the efficiency of the electric telegraph depends upon the breaking and renewing the current of electricity by means of which a succession of waves of electricity are sent through the conducting wires. Now it has been ascertained that three electric waves may travel simultaneously through the wires of the Atlantic telegraph with sufficient intervals between them to record the indications they are intended to convey; that is, three signals can be intelligibly and practically transmitted in two seconds.
The original design, structure, and difficulty of depositing the cable are only equalled by the talent and perseverance with which it has been done. The 5th of August, 1858, will be memorable for the accomplishment of the boldest enterprise that ever was undertaken by man, and which is only the beginning of a vast submarine communication that will ultimately encircle the globe. It has been granted to British genius thus to annihilate time and space, in order to connect all mankind into one great family for their moral and religious advancement; and, whatever may be the fate of the British Islands in the course of ages, to their energetic race the glory will remain of having been the chief instruments in the hands of Providence for the civilization of the world—a civilization which will extend with the development of their numerous colonies into great independent Christian states, like those of the Union in North America. The thunderbolt snatched from heaven by Franklin now passes through the depths of the Atlantic as a messenger of peace between the kindred nations.[[16]]
In telegraphs on land the intensity of the battery or magnets is increased by induction on the same principle. It is by intensity that the electric current is enabled to pass through the wires, and that is augmented by increasing the number of coils round the cylinder: however, it is only advantageous when the distance between the stations is great, for then the resistance in the additional coils bears a small proportion to the resistance offered by very long wires, but a very great proportion to that opposed in very short ones. The nice adjustment for each case has been determined by the experiments of eminent electricians, and all the arrangements have been brought to great perfection in this wonderful triumph of science, which is due to Volta, who called into existence the fiery stream, and to Faraday, who has given it the energy of the lightning.
When the length of the wire in the helices of an electro-magnet is very great, it offers increasing resistance to the passage of the electricity, so that the cessation of magnetism is not instantaneous when the contact with the Voltaic battery is broken. To remedy that defect an instrument has been invented which instantaneously deprives the apparatus of the remaining electricity. A great length of fine wire gives the severest shocks, while a shorter and thicker wire gives the longest sparks and ignites the greatest quantity of platinum wire.
Ruhmkorff’s electro-inductive apparatus has either been improved, or new machines constructed, by Messrs. Grove, Gassiot, and Joule, of intense energy. Indeed, so great is the energy of electro-induction, that hopes were entertained of its superseding steam as a motive power. For the current of electricity from an electro-magnet can be made to flow in opposite directions, so as to produce alternate attractions and repulsions, and consequently a continued motion, which might be applied as a motive force to machinery. However, Mr. Joule has proved that the power developed by one pound of coal in combustion is to that produced by one pound of zinc consumed in Mr. Grove’s powerful electro-magnetic apparatus as nine to one, so that, even if zinc were as cheap as coal, and a Voltaic battery as easily kept in order as an engine-furnace, electricity will not supersede steam as a motive power.
A current of electricity traversing a conductor gives out a quantity of heat determined by fixed laws, the amount of which is invariable as long as the machine to which it is applied remains at rest; but the instant the machine is set in motion a reaction takes place in the intensity of the current, causing a diminution in the quantity of heat, because the heat that disappears is converted into the mechanical force exerted by the engine.