The relay was connected through its keys to another local battery.
If the key was closed—that is, pressed down so as to form a metallic connection—the relay magnets were magnetized by its local battery, and the little needle was drawn toward them by their attraction, until the short arm of the needle rested against the little stud. This touching of the needle to the stud closed the circuit of the second local battery, and the sounder armature answered to the influence. If the key was opened, the circuit was broken, the needle was drawn back by its little spring and the local current of the sounder disrupted. With the exception of the needle, the whole apparatus was precisely similar to that employed in telegraphing in 1887.
Cobb examined it carefully and noted its delicacy and the care exercised in its protection from external forces by being covered with a glass globe and surrounded by helices in opposite directions.
Mr. Doane watched his expression, and smiled at his perplexity.
“Simple, isn’t it?” he asked.
“Yes, in construction,” returned Cobb; “but its theory of action upon the distant instrument is to me a total mystery, I must confess.”
“But easily explained, in so far as how it acts, but not why it performs its work,” Mr. Doane answered. “The needle which you see has a mate, and that mate is in the California office. These needles are made in pairs, and, by a wonderful process, made sympathetic. No two pairs are charged with the same sympathy; consequently, no other needle of the whole system of instruments will affect this save that single one in California. The instruments of each pair are most carefully set up at their different stations, so that the needles shall point to the true north; thus the needles are exactly parallel to each other. When the instrument is not in use, the key is left open, and the needle is held back by its spring. Now, if the California operator should close his key, he would cause his needle to be attracted toward the relay magnets; this movement of his needle exerts a sympathetic influence upon the needle in this instrument. It endeavors to parallel itself to its mate. It moves to the right, overcoming the power of its spring, and, touching the stud, closes the circuit, and the sounder records the fact. Opening his key in California, both needles move back by the tension of their springs, and the sounders are demagnetized. The sympathy of these two needles to place themselves in a parallel position, or, more properly, the repulsion of the poles of each from those of the other, is the secret of the sympathetic telegraph system.”
“A wonderful, grand invention!” burst from the lips of Cobb, as he comprehended the almost human action of the two needles. “How could mortal man have discovered such a secret of nature!”
“Yes, Junius; it is wonderful!” echoed Hugh.
“How many pairs of these sagacious little instruments have you in the system?” asked Cobb, after a silence.