Page:The Granite Monthly Volume 1.djvu/115

Rh or three inches in diameter, shaped like a spool or bobbin, and wound with several thousand turns of small wire, which is wound with fine silk to keep the metal from coining in contact. It is wound or coiled exactly like a bundle of new rope, a small hole being left in the middle a little larger than a common wooden pencil. In the centre of this is suspended a very thin, delicate mirror, about as large over as a kernel of corn, with a corresponding small magnet rigidly attached to the back of it. The whole weighs but little more than a grain, and is suspended by a single fibre of silk much smaller than a human hair, and almost invisible. A scale is placed two or three feet from the mirror, a narrow slit being cut in the centre of the scale to allow a ray of light to shine upon the mirror from a lamp placed behind said scale, the little mirror, in turn, reflecting the light upon the scale. This spot of light upon the scale is the index by which all the messages are read. The angle through which the ray moves is double that traversed by the mirror itself, and it is, therefore, really equivalent to an index four or six feet in length without weight. To give an idea of the extreme delicacy of this apparatus, I may state that messages have actually been sent by means of a common percussion gun-cap fitted with a very small piece of zinc, excited to electrical action by a drop of acidulated water of the simple bulk of a tear.

When the operator at Rye Beach sends a message, each word is spelled out in full, in the ordinary way, by tapping the double "key" before mentioned. The right hand strokes correspond to the dashes, and the left hand strokes to the dots of the ordinary land telegraph. [The cable code varies a little from the Morse code.] When the operator at Rye Beach presses down the right hand "key," he causes a current of electricity to flow, feeble though it is, in a certain direction, which passes through the coil of the galvanometer at Torbay (600 miles distant), which affects the magnetism of the little magnet attached to the mirror and causes it to "deflect" or turn to the right. When the left hand "key" is pressed clown, the current circulates in the opposite direction, and, obviously, causes the little mirror to move in the opposite direction. Of course, when the mirror moves, the ray of light moves with it. By this means the right and left hand strokes—which represent the dashes and dots—are obtained, thus enabling the operator to read the message.

To the casual spectator, there is nothing but a thin ray of light darting about with irregular rapidity; but to the trained eye of the operator every flash is replete with intelligence. Thus the word "boy," already alluded to, would be read in this way; One flash to the right and three to the left is B. Three flashes to the right is O. One to the right, one to the left and two more to the right is Y, and so on. Long and constant practice makes the operators expert in their profession, and enables them to read from the mirror as readily and as accurately as from a book or a newspaper. The galvanometer used is the invention of Sir William Thomson of Scotland, who is also the inventor of several other splendid instruments used in telegraphy, principally for "testing" purposes, and consequently of the most delicate kind.

The "testing" of the cable is a wonderful and mysterious process, by which the electrician can sit at a table on either side of the ocean and in an hour "locate" the exact spot where the conducting power of the cable may be in the slightest degree impaired. It does not always follow that the cable is broken, or even very badly fractured, when the electric current is interrupted. It may be that the "insulation" is imperfect; for the conducting power of a cable depends upon the perfection of its "insulation"—that is, its ability to retain the whole of the current sent over the conducting wire, by being completely imbedded in a gutta percha covering, which is a non-conductor of electricity. Perhaps the gutta percha may have been scraped off by chafing on a rock. It may have been cracked by receiving some unusual tension. It may have been ruptured, if near the shore, by the grapnel of some boat or vessel. Possibly some prying fish may have tried his teeth upon it. Any of these or similar causes would be