As the copper strand was prepared, it was rolled upon drums, and then taken from the drums to have three separate coatings of gate percha applied, until the aggregate diameter was thus brought up to about three-eighths of an inch. The gutta percha used for these coatings was prepared with great care. It was first rasped into shreds, and washed, and next pressed through several layers of fine wire gauze, and kneaded for hours in the interior of iron cylinders by steam machinery. It was then squeezed by poWerful screws, through dies, as the strand of copper was gradually drawn along between them, and so made to adapt itself as a compact sheath to the strand. Three several and successive coatings were given to the strand in order that any imperfection left in the first might be compensated and remedied by the next coat applied. The completed core was subjected to a pressure of five tons upon the square inch, by the use of hydrauli• power, without the insulating material being at all injured by the force applied.
During the process of the manufacture of this core it was submitted to constant examination to prove both that the continuity of tho copper strand continued unimpaired, and also that the insulating power of the gutta percha sheath was as complete as it was required to be. The continuity was proved by passing a voltaic current of low intensity from a battery of a single. pair of plates, through the strand, and then causing it to record a signal after issuing from the wire. A battery of low intensity wa employed for this purpose, because it made the test so much the more severe. A strong battery might have thrown the current through a slight imperfection, which a weak battery might not be able to overcome. The due perfection of the insulation was tried by turning up into the air the end of the length of core about to be examined, and by then connecting one pole of a voltaic battery of five hundred pairs of plates with the nearer end of the length of wire, and the other pole with the earth, a magnetic galvanometer being suspended within a coil continuous with the strand. So long as the insulation of the strand was fairly perfect, the copper wires became charged with the electricity of which but very little could escape, and so no current was produced through the strand, and no deflection of any consequence appeared in the magnetic needle. When the insulating sheath, on the other hand, was imperfect, the electrical charge leaked through the imperfections to the earth, and so got back to the opposite pole of the battery. In this way a current was set up in the wire to supply the leakage, and the magnetic needle was deflected from its position of equilibrium, the deflection being in proportion to the amount of the current. A strong battery of five hundred pairs of plates was employed in detecting imperfect iusula tion, in preference to a weak one, because a strong current would force a Talmage through an Imperfection which might be too alight to allow a weak current to make its way. During the progress of the work, a plan was devised which enabled the testing for both con tinuity of the strand and insulation of the sheath to be carried on simultaneously. A voltaic current can leas through a charged Leyden
jar without either the current or the charge being in any practical way interfered with. Therefore the entire length of cable under examination was joined up into a loop or endless ring, and a voltaio battery of five hundred pairs of plates had one of its poles connected with the conducting strand of this ring, and the other pole placed in communication with the earth. A small insulated battery of low tension was also introduced into the circuit of the ring, ao that its current flowed round continually, from pole to polo, through the strand. An insulated bell was also so placed in the circuit, that any break of continuity dropped a needle, before held magnetically fast, and caused the bell to sound. Another bell instrument was so arranged that it was rung whenever the current from the five hundred cell battery began to run, in consequence of electrical leakage, with undesirable speed. The feeble battery in the circuit rung its bell whenever the circuit was broken. The strong battery out of the circuit rung its bell whenever an outflowing current was set up through the strand, in consequence of the insulating sheath being unable to retain the charge.
During the prosecution of these experiments the discovery was made, that the insulating power of gutta percha is very materially affected by temperature. A high temperature seems greatly to impair its insula tiug capacity, and the recurrence of slew temperature speedily restores it to its original excellence. An opportunity was taken, when a single flake or tier of the completed cable was lying at the bottom of the receptacle in the yard of the manufactory at Greenwich, to watch the changes which the natural variation of temperature during forty-eight hours produced in its conducting capability. When the thermometer stood at 42', the deflection of the galvanometer needle was barely 3° ; but when the thermometer rose to 59°, the deflection of the magnetic needle became 64'. Even passing sunshine and cloud made the tell tale needle traverse out and in with surprising rapidity. There was reason to conclude, from the soundings taken by Lieutenant Hayman of the Cyclops, that the bottom of the Atlantic would supply the low temperature essential to the good performance of the insulating material The separate lengths of manufactured core were joined into longer extents in the following manner. The gutta percha was scraped from the ends for a short distance, and these were placed in contact. A piece of copper wire was then attached by firm brazing to one side of the joint, and wound round the strand until it reached as far on the other side, being there brazed again. A second binding was then effected outside the first. in precisely the same way, and several layers of gutta percha placed over the whole by the aid of hot irons. In case of the core on each side of the joint being at any time so dragged that the ends of the strand were broken asunder, this outer investment of wire would unroll spirally without being detached from the strand. Thus the electric continuity of the strand would be preserved even when the strand itself was severed.