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Conductivity Of Incandescent Carbon Filaments, And Of The Space Surrounding Them

The first part of this paper, which relates to the conductivity of carbon filaments, is in the nature of a discussion of a paper read before the Institute by Prof. Anthony, at the May meeting in 1887. Prof. Anthony spoke of a change from negative to positive of the temperature coefficient of some carbon filaments which he had observed. He illustrated the change by a figure, which I will reproduce here for reference (Fig. 1). Prof. Anthony stated that this change did not occur in all filaments, and that he was unable to explain the circumstances under which it did occur. Prof. Elihu Thomson, in his discussion of Prof. Anthony’s paper, said that he had observed the same phenomenon, but had not investigated it sufficiently to explain its cause. About four years ago an inventor offered to the General Electric Company a method of coating carbon filaments with metal, claiming thereby a very great improvement in the performance of the filaments. The metallic coating was applied by heating the filament to high incandescence in the presence of the vapor, which was obtained from a very thick dark colored liquid, the application being made by the same means as is employed in the ordinary hydrocarbon flashing process. The proof which he gave that his filaments had a metallic coating was the fact that the resistance of these filaments was lower at about red heat than it was at higher temperatures. In order to test his assertions, I treated a number of filaments with his dark liquid, and, for a comparison, treated in exactly the same manner a number of similar filaments, using ordinary gasoline. To the surprise of both of us, the gasoline filaments showed the same rise in resistance, after passing the dull red temperature, as was shown by the filaments supposed to have a metal coating. This fact led me to investigate this phenomenon.