Engineering Wonders of the World
Volume I

EARLY ATLANTIC CABLES. 293 extra length of cable made for the coming expedition was subjected to searching tests for the conductivity of the copper wire, every length being rejected that fell below a certain value. At this period Professor Thomson also devised the mirror signalling instrument, which entirely revolutionized long distance teleg- raphy through cables. This ingenious invention consists of a small and exceedingly light steel magnet, M (Fig. 15), with a tiny mirror fixed to it, both together weighing little over a grain. This delicate re- Fig. 15.—THE MAGNET AND MIRROR OF A REFLECTING GALVANOMETER. fleeting magnet is suspended from its centre by a filament of silk and surrounded by a coil of the thinnest insulated copper wire. A very weak current is sufficient to pro- duce a slight, scarcely perceptible, movement Fig. 16.—REFLECTING GALVANOMETER AND SPEAKER. of the suspended magnet when electricity passes through the surrounding coil. A fine ray of light from a shaded lamp (see Fig. 17) at a short distance is directed through a slot in the screen shown, thence to the open centre of the coil A upon the mirror, which reflects it back to a graduated scale B. As may be inferred from the illustration, an exceedingly slight angle of deflection of the magnet A is thus made to magnify the movement of the spot of light upon the scale B sufficiently for ready and accurate observa- tion by the operating clerk. By a combination of these movements of the speck of light, Morse alphabet letters and words are read off as sent by a key at the transmitting end of the cable. Scale Fig. 17.—DIAGRAM TO ILLUSTRATE THE PRINCIPLE OF THE REFLECTING GALVANOMETER.