Early in the history of electric lighting it became apparent that the proper construction of the carbon electrodes was a highly important item in the manufacture of a lighting apparatus. The value of carbons depends largely upon their purity and freedom from ash in burning, and it required a countless number of experiments to develop the highly efficient carbons now in general use. Davy made use of pieces of wood charcoal in his experiments, but these were too fragile to be of practical value, even if their other qualities had been ideal. Later experimenters tried various compounds, and in 1876 Carré in France produced excellent carbons made of coke, lampblack, and syrup. From these were developed the present carbons, usually made by mixing some finely divided form of carbon, such as soot or lampblack made from burning paraffin or tar, with gum or syrup to form a paste. Rods of proper size and shape are made by forcing this paste through dies by hydraulic pressure, subsequently baking them at a high temperature. Sometimes they are given a coating of copper, a thin layer of the metal being deposited upon them by electrolysis.
EDISON AND THE INCANDESCENT LAMP
The familiar incandescent electric-light bulb seems such a simple apparatus to-day, being nothing apparently but a small wire enclosed in an ordinary glass bulb, that it is almost impossible to realize what an enormous amount of money, energy, and that particular quality of mentality which we call "genius" has been required to produce it. First and foremost among the names of the men of genius who finally evolved this lamp is that of Thomas A. Edison; and only second to this foremost name are those of Swan, Lane-Fox, and Hiram Maxim. But Edison's name must stand preeminent; and there are probably very few, even among Europeans, who would attempt or wish to deny him the enviable place as the actual perfecter of the incandescent-light bulb.
THOMAS A. EDISON AND THE DYNAMO THAT GENERATED THE FIRST COMMERCIAL ELECTRIC LIGHT.
It is said that Edison first conceived the idea of an incandescent electric light while on a trip to the Rocky Mountains in company with Draper, in 1878. Be this as it may, he certainly set to work immediately after completing this journey, and never relaxed or ceased his efforts until a practical incandescent lamp had been produced. His idea was to perfect a lamp that would do everything that gas could do, and more; a lamp that would give a clear, steady light, without odor, or excessive heat such as was given by the arc lights—in short, a household lamp.
Early in his experiments he abandoned the voltaic arc, deciding that a successful lamp must be one in which incandescence is produced by a strong current in a conductor, the heat caused by the resistance to the current producing the glow and light. But when search was made for a suitable substance possessing the necessary properties to be the incandescent material, the inventor was confronted by a vast array of difficulties. It was of course essential that the substance must remain incandescent without burning, and at the same time offer a resistance to the passage of the current precisely such as would bring about the heating that produced incandescence. It should be infusible even under this high degree of heat, or otherwise it would soon disappear; and it must not be readily oxidizable, or it would be destroyed as by ordinary combustion. It should also be of material reducible to a filament as fine as hair, but capable of preserving a rigid form. These, among others, were the qualities to be considered in selecting this apparently simple filament for the incandescent lamp. It was not a task for the tyro, therefore, that Edison undertook when he began his experiments for producing an "ideal lamp."
The substance in nature that seemed to possess most of the necessary qualities just enumerated was the metal platinum, and Edison began at once experimenting with this. He made a small spiral of very fine platinum wire, which he enclosed in a glass globe about the size of an ordinary baseball. The two ends of the wires connected with outside conducting wires, which were sealed into the base of the bulb. The air in the bulb had to be exhausted and a vacuum maintained to diminish the loss of heat and of electricity and to prevent the oxidation of the platinum. But when the current was passed through the spiral wire in this vacuum a peculiar change took place in the platinum itself. The gases retained in the pores of the metal at once escaped, and the wire took on such peculiar physical properties that it was supposed for a time by some physicists that a new metal had been produced. The metal acquired a very high degree of elasticity and became susceptible of a high polish like silver, at the same time becoming almost as hard as steel. It also acquired a greater calorific capacity so that it could be made much more luminous without fusing. To diminish the loss of heat the wire was coated with some metallic oxide, and the slope of the spiral also aided in this as each turn of the spiral radiated heat upon its neighbor, thus utilizing a certain amount that would otherwise have been lost. But despite all this, Edison found, after tedious experimenting, that platinum did not fulfil the requirements of a practical filament for his lamp; it either melted or disintegrated in a short time and became useless; and the other experimenters had met with the same obstacles to its use, and were forced to the same conclusion.