[SUBMARINE CABLES.]
THEIR MANUFACTURE.
Twenty-seven years ago the first submarine cable was laid across the Strait of Dover. This was a single copper wire covered in gutta-percha, which parted next day; and the first practicable submarine cable was laid in 1851, on the same route. Since then the progress of ocean telegraphy has been extraordinary; no fewer than six cables spanning the Atlantic bed—five to North America (although these are not all working), and one to South America by way of Madeira and Pernambuco. And so extensive is the already existing network of foreign cables, that when Asia is united to America by cabling the Pacific, the electric girdle round the world will be complete from east to west, as it now is between north and south.
In this great development of telegraphy our countrymen have unquestionably furnished both the lion's share of the work and the capital. The cables have nearly all been manufactured in London, which is the headquarters of telegraphy.
The principal parts of a submarine cable are: the conductor; the insulator; and the protector or sheathing. The conductor, as its name implies, is the wire which conducts or conveys the electric current from one place to another. It corresponds to the iron wire of our ordinary open air or land lines of telegraph. Along this wire, as is well known, the current from the battery at the station from which the message is being sent travels to the station receiving the message, where it passes to the earth, and appears to return through the earth to the battery again; thus completing its circuit. There are two distinct parts of the circuit which the current has to traverse—namely, the outgoing part, represented by the wire or conductor; and the return part, represented by the earth itself; and inasmuch as these two parts must be kept distinct and apart throughout their length, the wire which is laid along the earth's surface must be kept apart from the earth, to secure which the conductor is entirely surrounded with an insulator. In land lines, erected on posts overhead, the wire is separated or insulated from the earth by the air, which is, when dry, the most perfect insulator known; and at the points of support, contact with the earth is prevented by the use of porcelain, stoneware, or vulcanite 'insulators,' to which the conducting wire is fastened.
An insulator is a non-conducting substance, impervious, so to speak, to electricity. It is the theoretical antipodes of a conductor. While the conductor is a substance through or over which electricity flows freely, the insulator will neither permit electricity to pass through its mass nor over its surface. It can therefore be used as a means of confining electricity to a conductor, and preventing it from escaping to other conductors in the neighbourhood. In short it can be made to insulate or isolate the particular conductor from all other conductors. Its use in a submarine cable is to confine the electric current to the conductor or wire, so that it travels along it from one station to the other without escaping to the water, and through that to the earth (which, as we have already said, is the neighbouring conductor, and the return part of the circuit) on its way. It is therefore of course important that there shall be no flaw in the insulator, and in order to protect it from strain and violence, it is covered with a strong guard or sheathing. This outer sheathing or protector, which is composed of twisted metal strands, is purely mechanical. Only the conductor and the insulator are concerned in the electrical requirements of the cable.
The conductor is invariably of copper wire, that metal being chosen because, next to silver, which is of course too expensive, it is the best metallic conductor of electricity. The metals, as distinguished from most other minerals, are excellent conductors of electricity; that is to say, they oppose relatively less resistance to the passage of the electric current through their mass. There is an economy of power in using a good conductor for the telegraphic line. The current is less weakened when the resistance to its passage along the line of wire is less, and it is therefore capable of more powerful effects throughout the route, and consequently at the other end. The conductivity or conducting power of a wire increases with the thickness of the wire; and therefore by taking a thicker wire of a more common metal than copper (such as iron), the resistance to the passage of the current may be made as small as when a thin copper wire is employed. But it is important that the conductor of a submarine cable, especially of a long one, should be of as fine dimensions as possible, in order to economise insulating material and sheathing, and reduce the total weight of the finished line. Therefore the advantages in point of price of iron wire over copper in the first place, would be greatly overbalanced by the increased cost of insulating and sheathing it. It is of the greatest importance that the copper wire of the cable should be as pure as possible, for the slightest trace of arsenic or other foreign element is sufficient to hamper, in some mysterious way, the swift course of the subtle current, and very materially to weaken the conducting power of the wire.
In a few cables the copper conductor has been made in the form of a single thick wire; but for the sake of greater flexibility and less risk of breakage, it is generally made in the form of a strand of three or more, and frequently of seven wires; six set round a central one. The wires are wound together in a spiral strand, and their interstices filled with an adhesive substance called Chatterton's Compound, a mixture of resin, gutta-percha, and Stockholm tar. This compound not only renders the strand solid, and impervious to water, but also acts as an adhesive connection between the copper conductor itself and the insulator with which it is to be coated. Bound together with this or similar pitchy compounds, the conductor and the insulator form a solid core expanding and contracting together.
The insulator is always either of gutta-percha or india-rubber, but most frequently the former; and it is of course essential that there shall be no flaw or defect, such as an air-bubble or steam-vesicle, or hair or thread inclosed so as to deteriorate its insulating properties. To guard against such accidents, it is usual to apply a series of coatings to make up the total thickness of the insulator. Accordingly, when one coating has cooled, a layer of Chatterton's Compound is applied to it, and another coating overlaid, and so on, until the required amount of insulating material has been put on.