On passing a current through the carbon the small rod is heated to incandescence, and imparts heat to the surrounding mass.
- 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.
Edison in 1878 again attacked the problem of producing light by the incandescence of platinum.
The perfectly pure metal may be prepared by heating the oxide or oxalate in a current of hydrogen; when obtained at a low temperature it is a black powder which oxidizes in air with incandescence; produced at higher temperatures the metal is not pyrophoric. Peligot obtained it as minute tetragonal octahedra and cubes by reducing ferrous chloride in hydrogen.
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.