- Independently of the question of the application of external heating, the furnaces used in electrometallurgy may be broadly classified into (i.) arc furnaces, in which the intense heat of the electric arc is utilized, and (ii.) resistance and incandescence furnaces, in which the heat is generated by an electric current overcoming the resistance of an inferior conductor.
On passing a current through the carbon the small rod is heated to incandescence, and imparts heat to the surrounding mass.
The temperature of the electric furnace, whether of the arc or incandescence type, is practically limited to that at which the least easily vaporized material available for electrodes is converted into vapour.
It is not necessary that all electric furnaces shall be run at these high temperatures; obviously, those of the incandescence or resistance type may be worked at any convenient temperature below the maximum.
Such spectra seem to be characteristic of complex molecular structure, as they appear when compounds are raised to incandescence without decomposition, or when we examine the absorption spectra of vapours such as iodine and bromine and other cases where we know that the molecule consists of more than one atom.
It combines with fluorine with incandescence at ordinary temperatures, and with chlorine at 250-300°; carbon, silicon, and boron, when heated with it in the electric furnace, give crystals harder than the ruby.
Edison in 1878 again attacked the problem of producing light by the incandescence of platinum.
Edison in the United States, were engaged in struggling with the difficulties of producing a suitable carbon incandescence electric lamp. Edison constructed in 1879 a successful lamp of this type consisting of a vessel wholly of glass containing a carbon filament made by carbonizing paper or some other carbonizable material, the vessel being exhausted and the current led into the filament through platinum wires.