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.{82}
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.
