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.

Whether the insulating substance is gutta-percha or india-rubber, there is generally wound round it a serving of untarred hemp or jute yarn, which has either been tanned or soaked in brine as a preservative. This is to act as a padding or cushion for the iron sheathing or protector next to be applied. This serving is applied in the following way. A circular disc or frame, carrying on one face a series of bobbins which hold the threads of the yarn, is kept revolving. The core is made to pass through a hole in the centre of this disc, and the threads are wound spirally round it as the disc revolves.

The iron wires of the sheathing, which completely inclose and cover the served core, are wound on by the powerful 'cable machines,' whose operation is so interesting a feature in a visit to a cable factory. The great revolving disc, seven or eight feet in diameter, is set round with iron bobbins filled with the iron sheathing wires. These bobbins are suspended on the face of the disc, so that as the disc revolves they always preserve their fixed position with respect to the earth. In this way the wires themselves are not twisted round their own axes as they are laid on the core. These wires are generally of the best homogeneous iron wire, that is, a wire intermediate in quality between iron and steel, and uniting some of the toughness of the former to the strength of the latter. They are sometimes themselves covered with a serving of the best tarred Manilla hemp; sometimes laid on in single wires abutting against each other, so as to form a smooth and complete casing for the cable; and sometimes they are applied in strands of three wires, each abutting against each other. The composite sheathing of hemp and iron is usually applied to the deep-sea portion of a cable where, in laying, a union of lightness and strength is demanded, and where, when once laid, the cable is not likely to be molested. The single-wire sheathing is applied to cables to be laid in shallower depths, such as coast-waters; and the heavy-strand sheathing is for protecting the cable in anchorages and on sea-beaches. The light-sheathed cable is called 'main' or 'deep-sea cable;' the medium is called 'intermediate;' and the heavy-sheathed cable is called 'shore-end.' There is seldom more of the last than ten or twelve miles, to carry the cable well out of reach of the abrasion of storm-shifted boulders and coast anchorage. The intermediate usually extends until deep water has been reached, where the deep-sea portion takes its place. These three types of cable are connected together by 'taper pieces.' The core is of course uniform throughout the entire length of the cable; but the taper pieces serve to connect the different types of sheathing artistically and soundly with each other. The intermediate cable generally has its sheathing wires covered with a serving of mineral pitch, silica, and hemp of a coarse quality, in order to ward off as long as possible the dissolving action of the sea-water.

The cable being thus finished at the manufactory, it is coiled into large iron tanks, and there immersed in brine until it is shipped for transport and laying.

All the materials of a submarine cable are carefully watched and tested—the iron wire, for stretching, twisting, and breaking stress, and the core for all its electric properties. The special properties of every knot or mile of the core are chronicled, so that a complete account of every part of the cable is preserved during its progress of manufacture. And after it is made, it is tested electrically every day, to see that no change takes place in its electric qualities. These electric tests are three in number: For resistance—the resistance of the copper wire to the passage of the current. For inductive capacity—the amount of charge or quantity of electricity which the cable will take up. For insulation resistance—the insulating power of the gutta-percha coating.

These tests are made by direct comparison with units, just as bodies are weighed by comparison with a pound or unit of weight. The unit of electric resistance is the ohm; so called after the celebrated German physicist and electrician Ohm. The ohm is the resistance of a certain length of platinum-silver wire determined by a Committee of the British Association. Multiples of the ohm are readily obtained, and these are arranged and made up into what are called resistance-boxes—the practical tool of the electrician. A resistance-box usually contains coils of platinum-silver wire of from five thousand ohms downwards to single ohms or fractions of an ohm. It is with this finely graduated tool that the electrician compares the resistance, or in other words ascertains the conducting power of the cable.

The unit of measurement of the insulation resistance of the cable is a very high multiple of the ohm, called the meg-ohm or million-ohms; for inasmuch as the insulator is, technically speaking, a non-conductor, its office is to exercise the necessary resistance to the escape of the current. The unit for capacity is called a micro-farad or millionth part of the Farad, which derives its name from Faraday, and represents a certain quantity of electricity. The practical tool for the micro-farad is a contrivance called a condenser, a description of which, without the aid of drawings, would be too technical for our readers. A submarine cable is itself, however, a particular form of such a condenser. The copper wire is one of the opposed conductors, the sheathing, earth, and sea-water form the other, and these are separated from each other by the insulating coating of gutta-percha. It is a curious fact that when a charge of electricity is communicated to the copper wire of a cable, it induces a charge of an opposite kind in the earth outside. This inductive property of an insulated wire contiguous to the earth has an important bearing on practical telegraphy; for inasmuch as the communicated charge and the induced charge attract each other, the former travels less swiftly along the wire; it is held back, as it were, by the retarding influence of the earth's induced charge; or in other words has a tendency to ooze out of the cable instead of travelling uninterruptedly to the other end. It is of consequence, therefore, to ascertain the inductive capacity of a cable; as the less it is, the greater will the speed of signalling be.

The resistances and capacity of a cable are usually tested, according to the standards of resistance and capacity—that is, with the ohm, meg-ohm, and micro-farad—by measuring the strength of an electric current passing through the cable, by means of an instrument called the galvanometer, or current measurer. Its principle depends upon the fact, discovered by Oersted, the famous Copenhagen philosopher, that when a current is sent along a wire in the neighbourhood of a freely suspended magnetic needle, the needle will be deflected into a new position, and this position will be to right or to left according as the current of one kind or the other is sent through the wire. Moreover, the amount of deflection will be directly proportional to the strength of the current. This great discovery, which gave an incalculably great impetus to the progress of the telegraph, is the theoretical basis of the galvanometer. One form of this instrument, used to test submarine cables, is called the 'reflecting galvanometer,' and is the invention of Sir William Thomson. The wire through which the current to be measured is made to pass, consists of a great many turns of silk-covered or insulated copper of a very fine gauge, forming a hollow coil, in the heart of which a very diminutive magnetic needle is suspended by a gossamer-like filament of floss silk. This magnet (or magnets, for there are generally more than one) carries a tiny circular mirror, the whole arrangement of magnets and mirror being no longer than (=) [Transcriber's note: The parallel lines in the original shown here by = were 2mm (1/16") apart and 3mm (1/8") long.]. A beam of light is thrown from a lamp in front on to the mirror, and reflected back again on to a graduated pasteboard scale. When the current to be measured is sent throughout the coiled wire surrounding the magnets, they are turned horizontally on their former position, and the mirror is inclined round with them, so that the reflected beam of light is moved along the scale, the distance to which it is moved being a measure of the current strength.

Now when the current from a given battery or source of electricity is made to pass through wires of different resistances, the strength of the current which will pass through these wires can be made a measure of their resistances; and therefore, when the current from a particular battery is sent through the conductor of the cable or to test the insulator, and in each case measured by the galvanometer, and compared with the current from the same source which will flow through the units of comparison, the copper resistance and insulation resistance can be obtained.

In a somewhat similar way the capacity—the amount of electricity which a cable will take—is compared with the capacity of a standard condenser or measure of capacity. The opposite plates or sheets of the condenser are charged by a particular battery; and as these charges are eager to flow into each other and unite, but are held apart by the insulator, they may be allowed to do so through a wire or other conductor. The discharge of the opposite electricities into one another sets up a short powerful current in this wire, and its strength is proportional to the quantity of electricity discharged; that is, to the capacity of the condenser. If the coil of the galvanometer be substituted for this discharging wire, the strength of this discharge will be measured by the deflection of the gleam of light on the scale. By charging alternately, therefore, the condenser and the cable from the same battery, and observing their respective discharges by means of the galvanometer, the capacities of the cable and condenser are compared.

The speed of signalling through a submarine cable, that is to say the number of words per minute that can be transmitted through it, varies with the resistance of its conductor, its inductive capacity, and its length; and it is by a consideration of these properties, together with weight and cost of material, that its form and dimensions are designed; and on this interesting subject we may have a few words to say in a future paper.