“Yes, my boy, I do,” answered Smith. “The lengths of the cable in the three tanks are joined up into one length, and telegraphing—for the purpose of testing it—has been kept up with the shore without intermission from the moment we left Ireland, and began to pay out. It will be continued, if all goes well, until we land the other and in Newfoundland. The tests are threefold,—first, for insulation, which, as you know, means the soundness and perfection of the gutta-percha covering that prevents the electricity from escaping from the wires, through the sea, into the earth; secondly, for continuity, or the unbroken condition of the conductor or copper core throughout its whole length; and, thirdly, to determine the resistance of the conductor, by which is meant its objection to carry our messages without vigorous application of the spur in the form of increased electrical power in our batteries. You see, Robin, every message sent to us from the shore, as well as every message sent by us in reply, has to travel through the entire length of the cable, namely about 2400 miles, and as every mile of distance increases this unwillingness, or resistance, we have to increase the electrical power in the batteries in proportion to the distance to which we want to send our message. D’you understand?”
“I think I do, sir; but how is the exact amount of resistance tested?”
Mr Smith smiled as he looked at the earnest face of his young questioner.
“My boy,” said he, “you would require a more fully educated mind to understand the answer to that question. The subtleties of electrical science cannot be explained in a brief conversation. You’ll have to study and apply to books for full light on that subject. Nevertheless, although I cannot carry you into the subject just now, I can tell you something about it. You remember the testing-room which I showed you yesterday—the darkened room between the captain’s state-room and the entrance to the grand saloon?”
“Yes, sir, I remember it well,” responded Robin,—“the room into which the conducting-wires from the ends of the cable are led to the testing-tables, on which are the curious-looking galvanometers and other testing machines.”
“Just so,” returned Smith, pleased with his pupil’s aptitude. “Well, on that table stands Professor Thomson’s delicate and wonderful galvanometer. On that instrument a ray of light, reflected from a tiny mirror suspended to a magnet, travels along a scale and indicates the resistance to the passage of the current along the cable by the deflection of the magnet, which is marked by the course of this speck of light. Now, d’you understand that, Robin?”
“I—I’m afraid not quite, sir.”
“Well, no matter,” rejoined Smith, with a laugh.
“At all events you can understand that if that speck of light keeps within bounds—on its index—all is going well, but if it travels beyond the index—bolts out of bounds—an escape of the electric current is taking place somewhere in the cable, or what we call a fault has occurred.”
“Ah, indeed,” exclaimed Robin, casting a serious look at the cable as it rose from the after-tank, ran smoothly over its line of conducting wheels, dropped over the stern of the ship and glided into the sea like an an endless snake of stealthy habits. “And what,” he added, with a sudden look of awe, “if the cable should break?”