A very weak current is sufficient to produce a slight, though nearly imperceptible, movement of the suspended magnet when electricity passes through the surrounding coil. A fine ray of light from a shaded lamp, behind a screen (Figs. 20 and 21) at a short distance, is directed through a slot in the screen, thence to the open center of the coil (c) upon the mirror. It is then reflected back to a graduated scale (f). As may be seen from Fig. 21, an exceedingly slight angle of motion on the part of the magnet (a) is thus made to magnify the movement of the spot of light upon the scale (f), and to render it so considerable as to be readily noted by the eye of the operating clerk. The ray is brought to a focus by passing through a lens. By combinations of these movements of the speck of light (in length and direction) upon the index, an alphabet is readily formed. The magnet is artificially brought back to zero with great precision after each signal by the earth’s magnetism, and also both by the natural torsion of the fiber and the controlling action of the adjusting magnet (e) (Fig. 20), with the{83} help of the thumb-screw (d) for regulation purposes.
In a word, Professor Thomson’s combined mirror-telegraph and marine galvanometer transmitted messages by multiplying and magnifying the signals through a cable by the agency of imponderable light.