He perceived at once that “permanence in the lamp and the subdivision of the light” were the two desiderata. He put the Voltaic arc aside as unsuitable, and addressed himself to the problem of obtaining the desired results from an incandescent solid. The subdivision of the light is really an important point, and a comparison between divided and undivided burners is in favour of the more diffused light in a number of burners. This subdivision Edison worked hard to secure, and, as it is said of him, “With a steadfast faith in the fulness of nature, a profound conviction that if a new substance were demanded for the carrying out of some beneficial project, that substance need only be sought for, he set to work.”
Mr. Edison found difficulties in his way. One was the apparent impossibility of illuminating by means of an incandescent solid, for even platinum will melt at a heat too low for use. But this apparent impossibility was overcome by the inventor’s genius. He, after many trials, found that if he raised the platinum to a white heat _in a vacuum_ he would practically obtain _a new metal_ which would sustain the required heat.
[Illustration: Fig. 273.—Edison’s platinum lamp.]
“In making an electric lamp without a regulator,” says Mr. Upton, “two things are essential,—great resistance in the wire, and a small radiating surface. Mr. Edison sought to combine these two essential conditions by using a considerable quantity of insulated platinum wire wound like thread on a spool.” This platinum, as shown in the accompanying cut (fig. 273), was suspended in a glass bulb _in vacuo_, the air contained in it being expelled by electricity, heating it, and suddenly cooling the platinum, and squeezing out the air by the process. But, after all, the great difficulty of the inventor was to insulate his wires so perfectly that they would not meet and become a conductor. For, to perfect his lamp, this non-conducting principle was a necessity, otherwise the current would flow across instead of going all along the wires. He had previously made many uses of carbon, which we know is infusible. He tried lampblack tar, but it contained air, and would not do.
Thread answered his purpose, but was too fragile and uneven in texture. It suddenly occurred to him that paper—_charred paper_—cut into a thread-like form would satisfy all his conditions.