_Alterations in the Electrical Apparatus._--Since the manufacture of the cable in 1857, Professor Thomson had become impressed with the conviction that the electric conductivity of copper varied greatly with its degree of purity. As a result of the professor's further investigations, the extra length of cable made for the coming expedition was subjected to systematic and searching tests for the purity and conductivity of the copper. Every hank of wire was tested, and all whose conducting power fell below a certain value rejected. Here, then, we have the first instance of an organized system of testing for conductivity at the cable factory--a system which has ever since been rigorously insisted on. _Professor Thomson's Mirror Instrument._--And now, in the spring of 1858, an invention was perfected that was destined to have a remarkable effect on submarine-cable enterprise. For Professor Thomson (now Lord Kelvin) devised and perfected the mirror-speaking instrument, then often described as the marine galvanometer,[26] of which it may be fairly said that it entirely revolutionized long-distance signaling and electrical testing aboard ship. This most ingenious apparatus consists of a small and exceedingly light steel magnet (_a_) (Fig. 19) with a tiny reflector or mirror fixed to it, both together weighing but a single grain or thereabouts. This delicate magnet is suspended from its center by a filament of silk and surrounded by a coil (_b_) of the thinnest insulated copper wire. [Illustration: FIG. 19.--The Reflecting Magnet.] 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 help of the thumb-screw (_d_) for regulation purposes. [Illustration: FIG. 20.--Reflecting Galvanometer and Speaker.] 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.