Page:Popular Science Monthly Volume 14.djvu/587

Rh with the gas-pipes, the circuit is incomplete and no current will flow; but if any part of the main, however distant from the battery, be connected with the adjacent gas-pipes, the circuit will be completed and the current will flow. Supposing our battery to be at Charing Cross, and our rod of copper to be tapped opposite Somerset House, a branch wire can be carried from the rod into the building, the current passing through which may be subdivided into any number of subordinate branches which reunite afterward and return through the gas-pipes to the battery. The branch currents may be employed to raise to vivid incandescence a refractory metal like iridium or one of its alloys. Instead of being tapped at one point, our main may be tapped at one hundred points. The current will divide in strict accordance with law, its power to produce light being solely limited by its strength. The process of division closely resembles the circulation of the blood; the electric main carrying the outgoing current representing a great artery, the gas-pipes carrying the return current representing a great vein, while the intermediate branches represent the various vessels by which the blood is distributed through the system. This, if I understand aright, is Mr. Edison's proposed mode of illumination. The electric force is at hand. Metals sufficiently refractory to bear being raised to vivid incandescence are also within reach. The principles which regulate the division of the current and the development of its light and heat are perfectly well known. There is no room for a "discovery," in the scientific sense of the term, but there is ample room for the exercise of that mechanical ingenuity which has given us the sewing-machine and so many other useful inventions, and which engages a greater number of minds in the United States than in any other nation in the world. Knowing something of the intricacy of the practical problem, I should certainly prefer seeing it in Mr. Edison's hands than in mine.

It is sometimes stated as a recommendation to the electric light, that it is light without heat; but to disprove this it is only necessary to point to the experiments of Davy, which show that the heat of the voltaic arc transcends that of any other terrestrial source. The emission from the carbon points is capable of accurate analysis. To simplify the subject, we will take the case of a platinum wire at first slightly warmed by the current, and then, through the gradual augmentation of the latter, raised to a white heat. When first warmed, the wire sends forth rays which have no power on the optic nerve. They are what we call invisible rays; and not until the temperature of the wire has reached nearly 1,000° Fahr. does it begin to glow with a faint, red light. The rays which it emits prior to redness are all invisible rays, which can warm the hand but can not excite vision. When the temperature of the wire is raised to whiteness these dark rays not only persist, but they are enormously augmented in intensity. They