CHAPTER II
THE MANUFACTURE OF THE LINE
Design and Construction—Ships—Testing, Shipment, and Stowage—Paying-out Machinery—Staff—Preparations for the Expedition.
THE construction of the cable was taken in hand the following February (1857).
The distance from Valentia, on the western Irish coast, to Trinity Bay, Newfoundland—the two landing-points selected[14]—being 1,640 nautical miles, it was estimated that a length of 2,500 N.M.[15] would be sufficient to meet all requirements. This would provide sufficient margin for a considerable amount of “slack” cable for accommodating the irregularities of the bottom. The Gutta-Percha Company of London were entrusted with the manufacture of the “core,” consisting of a strand of seven No. 22 B.W.G. copper wires (total diameter No. 14 gage) weighing 107 pounds per N.M. insulated, with three coatings of gutta-percha (to ⅜-inch diameter) weighing 261 pounds per N.M., the conductor being, in fact, covered to No. 00 B.W.G.
This formed a far heavier core than had been previously adopted, and on this account the difficulties of manufacture were proportionately increased.{47} The enormous pressure of the ocean at such depths involved also a much severer test for the core.
On the other hand, as we now know, the conductor—and consequently also the insulator—should have been still larger, to a material degree. The engineer of the line strongly urged a conductor weighing 392 pounds per N.M., with the same weight for the insulator;[16] but his fellow projectors (the business element of the undertaking) were all for getting the work done, while the weather permitted, that year; and they were perhaps overquick to recognize the difference in the capital required. Moreover, they were here supported technically by the views of the responsible electrician, as well as by such high authorities as Michael Faraday and Morse. The latter reported that “large coated wires used beneath the water or the earth are worse conductors—so far as velocity of transmission is concerned—than small ones; and, therefore, are not so well suited as small ones for the purposes of submarine transmission of telegraphic signals.” Faraday had stated: “The larger the wire, the more electricity was required to charge it; and the greater was the retardation of that electric impulse which should be occupied in sending that charge forward.”[17]
Thus it will be seen that although Faraday laid the foundations of a large proportion of the electrical engineering of to-day, his views in this instance did not prove to be correct. The theoretical{48} resemblance of a cable to a Leyden jar—in reference to the effect of charging either—seems to have been prominently in mind, without proper regard to the resistance offered by the wire to the electric current—a resistance which becomes less the greater the bulk of the wire. Besides the engineer being overridden in this matter, the word of the electrical adviser on the Board (Professor Thomson) regarding the carrying capacity or working speed that would be obtained with such a core as that decided on—in view of the length involved—was also unavailing.
While no one can fail to appreciate the businesslike manner in which this undertaking was pushed through from the moment of inception—comparing only too favorably with some experiences of to-day—it was, without doubt, a vast pity that more time was not devoted to a fuller consideration of some of the problems, such as that involved over the dimensions of the conductor and insulator. No serious fault could, however, be detected with its actual manufacture, though the methods of those days were primitive as compared with present practise, and a system of efficient electrical testing altogether wanting.
After various experiments had been made with sample lengths of different iron wires made up into cable, the contract for the outer sheathings was, in order to get through the work quickly, divided equally between Messrs. Glass, Elliot & Co., of Greenwich, and Messrs. R. S. Newall & Co., of Birkenhead—both originally pit-rope makers.
The insulated core was first surrounded with{49} a serving of hemp saturated with a mixture of tar, pitch, linseed-oil, and wax; and then sheathed spirally with an armor of eighteen strands, each containing seven iron wires of No. 22 B.W.G., the completed strand being No. 14 gage in diameter.