The covering of the conductor with its dielectric or insulating sheath was effected as follows:—The centre wire of the copper strand was first covered with a coating of gutta percha, reduced to a viscid state with Stockholm tar, this being the preparation known as “Chatterton’s Compound.” This coating must be so thick that, when the other six wires forming the strand were laid spirally and tightly round it, every interstice was completely filled up and all air excluded. The object of this process was two-fold; first, to prevent any space for air between the conductor and insulator, and thus exclude the increase of inductive action attendant upon the absence of a perfect union of those two agents, and, second, to secure mechanical solidity to the entire core; the conductors of some earlier Cables having been found to be to some extent loose within the gutta percha tube surrounding them, and thereby much more liable to permanent extension, mechanical injury, and imperfect centricity than those in which the preliminary precaution just described had been made use of. The whole conductor next received a coating of Chatterton’s Compound outside of it; this, when the core was completed, quickly solidified, and became almost as hard as the remainder of the subsequent insulation. It was then surrounded with a first coating of the purest gutta percha, which being pressed around it while in a plastic state by means of a very accurate die, formed a first continuous tube along the whole conductor. Over this tube was laid by the same process a thin covering of Chatterton’s Compound, for the purpose of effectually closing up any possible pores or minute flaws that might have escaped detection in the first gutta percha tube. To this covering of Chatterton’s Compound succeeded a second tube of pure gutta percha, then another coating of the compound, and so on alternately until the conductor had received in all four coatings of compound and four of gutta percha. The total weight of insulating material thus applied was 400 pounds to the nautical mile, against 261 pounds in the Cable of 1857-8.
The core, completed as described, and which had previously and repeatedly been under electrical examination, was at length submerged in water of a temperature of 75 deg. Fah., and so remained during twenty-four hours. This was done that the subsequent electrical tests for conductivity and insulation might be made under circumstances the most unfavourable to the manufacture, from the well-known fact, that the insulating power of gutta percha is sensibly decreased by heat. It also ensures uniformity of condition to the core under test, and, the temperature in which it was tested being higher by 20 deg. than that of the water of the North Atlantic, there was plenty of margin against any disappointment from the effects of temperature after submersion. At the expiration of the term of soaking, the coils of core submitted to that process were expected to show an insulation of not less than 5,700,000 of Varley’s standard units, or of 150,000,000 of those of Siemens’s standard. This of itself was a very severe test, but no portion of the core showed a less perfection than that of double of either of the above high standards.
Having passed this ordeal, and having been tested on separate instruments and by a different electrical process by the officers of the Atlantic Telegraph Company, in order to verify the observations of the contractors, the core was tested for insulation under hydraulic pressure, after which it was carefully unwound from the reels on which it had been wound for that purpose, and every portion was carefully examined by hand as it was rewound on to the large drums on which it was sent forward to the covering works at East Greenwich, to receive its external protecting sheath. It was then again submerged in water, and required once more to pass the full electrical tests above referred to. Finally, each reel of core was very carefully secured and protected from injury, and in this state was sent to East Greenwich, where it was immediately placed in tanks provided for it. In these it was covered with water, and the lids of the tanks being fastened down and locked, it remained until demanded for completion.
The manufacture and testing of the “core” of the Atlantic Cable having been completed at the Gutta Percha works as described, a telegraphic line was thereby produced which, without further addition of material or substance, beyond that of copper and gutta percha, proportionable to any required increase in its length, would be perfect as an electrical communicator through the longest distances and in the deepest water, in which element moreover it appears to be chemically indestructible, if the experience of some fourteen years may be taken as evidence. At this point, however, the final form to be assumed by the deep-sea Cable was subject to important mechanical considerations, which came into play across the path of those purely electrical; and upon the manner in which these considerations are met and dealt with, depend, not merely the primarily successful submersion, but the ultimate durability and commercial value of deep-sea Cables.
The problem in the case of the Atlantic Telegraph enterprise may be thus stated. Given a submarine telegraph core like that already described, constructed on the best known principles and perfect as to its electrical conductivity and insulation—it is required to lower the same through the sea to a maximum depth of two and a-half miles, so as not merely not to allow the insulating medium to be torn or strained, but so as not even to bring its normal elasticity into play against the more tensile but perfectly inelastic material of the conductor. For if the core were lowered into very deep water like that referred to without further protection, even supposing it to escape actual fracture by the adoption of extraordinary precaution and by the aid of fine weather, it is evident that whenever, as would be highly probable, either in the act of paying out, during the lifting or manœuvring of the ship, or even from the effects of its own weight, the gutta percha sheath became extended to the limit of its elasticity, the copper in the centre would be stretched to a corresponding extent, and, the tension being removed, the gutta percha in returning to its original length would pull back the now elongated copper, which thenceforward would in every such case “buckle up,” and exert a constant lateral thrust against the gutta percha; ending, probably, in its ultimate escape to the outside, and the consequent destruction of the core as an electrical agent. Moreover, in the event of an electrical fault being discovered in any submerged portion of the Cable during the process of “paying-out” in deep water, it is of paramount importance towards its recovery and repair, that the engineer should have such an assurance in the quality and strength of his materials as will enable him confidently to exert a known force in hauling back the injured part, without apprehension of damage to the vital portion of the Cable.
The solution of this question must therefore be found in adding mechanical strength externally to the core, by surrounding it with such materials and in such a manner as to relieve it from all that strain which it will unavoidably meet in depositing it in its required position. In the case of the original Atlantic Cable this was attempted by first surrounding the core with tarred hemp, which in its turn was enveloped spirally by eighteen strands of iron wire; each strand consisting of seven No. 22½ gauge wires. The entire weight of the Cable so formed was, in air 20 cwt. per knot, and in water 13·3 per knot. Being capable of bearing its own weight in about five miles perpendicular depth of water, and the greatest depth on the route being two-and a half miles, its strength was calculated at about as much again as was absolutely requisite for the work. This was thought at the time to be a sufficient margin, and certainly in 1858, owing to the greatly improved machinery employed, this Cable was payed-out with great facility and without undue strain, although portions of it had been lost by breaking during several previous attempts in the same summer. Subsequent investigation and experience, however, led to the conclusion, that in respect, not only to its mechanical properties, but especially with regard to its relative specific gravity, and to other points in its construction, the Cable of 1858 was very imperfect; and, with a view to ensure every practicable improvement in the structure of their new line, the promoters of the undertaking, so soon as they found themselves in funds, during 1863, issued advertisements with a view to stimulate inquiry into the subject, inviting tenders for Cables suitable for the proposed work. The specimens that were sent in, as the result of this public appeal, were submitted to the scientific advisers of the Company, who, after careful experiments with all the specimens, unanimously recommended the Atlantic Company to adopt the principle of the Cable proposed by Glass, Elliot, & Co., whose experience and success in this description of work are well known. The Committee, however, stipulated that they should settle the actual material of which the Cable was to be ultimately composed, and that these should be carefully and separately experimented on before finally deciding upon it; and in consequence of this stipulation upwards of one hundred and twenty different specimens, being chiefly variations of the principle adopted by the Committee, were manufactured and subjected to very severe experiment, as were also the various descriptions and quantities of iron, hemp, and Manilla proposed as components of these respective Cables. The result of it all was that the Committee recommended the Cable that was adopted as being, in their opinion, “the one most calculated to insure success in the present state of our experimental knowledge respecting deep-sea Cables,” taking care at the same time, by enforcing a stringent specification and constant supervision, to guard against any possible laxity in the details of its construction. The Cable so decided on weighed 35¾ cwt. per knot in air, but in water it did not exceed 14 cwt., being only a fraction heavier in that medium than the old Cable, though bearing more than twice the strain—the breaking strain of the new Cable being 7 tons 15 cwt. In water it was capable of bearing eleven miles of its own length perpendicularly suspended, and consequently had a margin of strength of more than four and a-half times that which was absolutely requisite for the deepest water. The core having been received from the gutta percha works, and carefully tested to note its electrical condition, was first taken to receive its padding of jute yarn, whereby the gutta percha would be protected against any pressure from the external iron sheath, which latter succeeded the jute. On former occasions this padding of jute had been saturated in a mixture of tar before being applied to the gutta percha; but experience had shown that this proceeding might lead to serious fallacies as to the electrical state of the core, cases having been repeatedly found where faults existed in the core itself—amounting to an almost total loss of insulation—which, however, were only discovered after being submerged and worked through, owing to the partial insulation conferred for a time upon the bad place by means of the tarred wrapping. The Atlantic core, therefore, was wrapped with jute which had been simply tanned in a solution of catechu, in order to preserve it from decay, and as fast as the wrapping proceeded the wrapped core was coiled into water, in which, not only at this stage, but ever afterwards until finally deposited in the sea, the Cable, complete or incomplete, was stored, and the water being able to freely pass through the tarred jute to the core, the least loss of insulation was at once apparent by the facility offered by the water to conduct away to earth the whole or a portion of the testing current.
The iron wire with which the jute cover was surrounded was specially prepared for this purpose, and is termed by the makers (Messrs. Webster & Horsfall) “Homogeneous Iron.” It was manufactured and rolled into rods at their works at Killamarsh, near Sheffield, and drawn at their wire factory at Hay mills, near Birmingham. This wire approaches to steel in regard to strength, but by some peculiarity in the mode of preparing it, is deprived entirely of that springiness which prohibits altogether the use of steel as a covering for the outsides of submarine cables. Ten wires were laid spirally round the core, and each of these wires was of No. 13 gauge, and was under contract to bear a strain of 850 to 1,100 lb., with an elongation of half an inch in every fifty inches within those breaking limits. The Cable, as completed and surrounded by these wires, had not the slightest tendency to spring, as would be the case if the metal were hard steel, and could be handled with great facility.
Before, however, these ten wires surrounded the core, each separate wire had to be itself covered with a jacket of tarred Manilla yarn, the object of which is at once to protect the iron from rust and to lighten the specific gravity of the mass, while adding also in some degree to the strength of the external portion of the Cable. The wire was drawn horizontally forward over a drum through a hollow cylinder, on the outside of which bobbins filled with Manilla yarn revolved vertically, and the yarns from these bobbins, being made to converge around the wire as it issued from the end of the cylinder, were thus spun tightly round the former. These Manilla-covered wires being wound upon large drums ready for use, the core, which we left some time back surrounded with jute, was passed round several sheaves, which conducted it below the floor of the factory, from whence it was drawn up again through a hole in the centre of a circular table, around the circumference of which were ten receptacles for ten drums, containing the Manilla-covered wire. Between these drums massive iron rods, fastened to the circumference of the table, rose, and converged around a small hollow cone of iron through the upper flooring of the factory, at a height of 12 or 14 feet above the table. The jute-covered core was pulled up vertically, and passed on straight through the hollow interior of the cone already mentioned, which latter formed the apex of the converging rods. This done, the ten wires from the ten drums were drawn up over the outside of the same cone, and as they rose beyond it converged around the core, which latter, being free from the revolving part of the machinery, was simply drawn out; while the circular table being now set revolving by steam power, the ten wires from the drums were spun in a spiral around the core, thus completing the Cable, which was hauled out of the factory by the hands of men, who at the same time coiled it into large iron tanks, where it was covered with water, and was daily subjected to the most careful electrical tests, both by the very experienced staff of the contractors and by the agents of the Atlantic Telegraph Company.
The distance from the western coast of Ireland to the spot in Trinity Bay, Newfoundland, selected as the landing-place for the Cable, was a little over 1,600 nautical miles, and the length of Cable contracted for, to cover this distance, including the “slack,” was 2,300 knots, which left a margin of 700 knots, to cover the inequalities of the sea-bed, and to allow for contingencies. On the first occasion 2,500 statute miles were taken to sea, the distance to the Newfoundland terminus on that occasion being 1,640 nautical miles; and, after losing 385 miles in 1857, and setting apart a further quantity for experiments upon paying-out machinery, sufficient new Cable was manufactured to enable the Niagara and Agamemnon to sail in 1858 with an aggregate of 2,963 statute miles on board the two ships, of which about 450 statute miles were lost in the two first attempts of that year, and 2,110 miles were finally laid and worked through.
The greatly increased weight and size of the Cable would have made the question of stowage a very embarrassing one had it not been for the existence of the Great Eastern steamship, there being no two ordinary ships afloat that would be capable of containing, in a form convenient for paying-out, the great bulk presented by 2,300 miles of a Cable of such dimensions. This bulk, and the now acknowledged necessity for keeping Cables continuously in water, made their influence to be felt in a very expensive manner to the Company and to the contractors throughout the progress of the work, even at this early stage. The works at Morden Wharf had to be to a very large extent remodelled to meet these contingencies. Eight enormous tanks, made of five-eighths and half-inch plate iron, perfectly watertight, and very fine specimens of this description of work, were erected on those premises, and these tanks then received an aggregate of 80 miles of Cable per week. Four of the tanks were circular in shape, and each contained 153 miles of cable, being 34 ft. in diameter and 12 ft. deep. The other four were slightly elliptical, being 36 ft. long by 27 ft. wide, and 12 ft. deep, and contained each 140 miles. The contents of all these, as they became full, were transferred to the Great Eastern at Sheerness, for which service the Lords of the Admiralty granted the loan of two sailing-ships, laid up in ordinary at Chatham, namely—the Amethyst and the Iris.[4] These ships had to undergo very considerable alteration to render them suitable for the work, portions of the main deck having to be removed—fore and aft—to make room for watertight tanks, which here, as elsewhere, were to be the medium for holding the Cable. The dimensions of the two tanks on board the Amethyst were 29 ft. diameter by 14 ft. 6 in. in depth, and each held 153 miles of Cable; of those on the Iris, one was 29 ft. diameter and 14 ft. 6 in. deep, and held 153 miles, and the other held 110 miles, and was 24 ft. wide, and 17 ft. deep.