MR. EDISON'S LAMP. 



269 



This application of electricity stirred up our transatlantic cousins, and 

 Mr. Edison was requested backed up by many influential persons to 

 make the investigation whether the light could be produced for domestic 

 purposes. The celebrated electrician undertook the commission, and cer- 

 tainly came unprejudiced to the encounter, for he had not at that time even 

 seen an electric light. 



He perceived at once that " permanence in the lamp and the sub- 

 division 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 impossk. 

 bility 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 ob- 

 tain a new metal which would sustain the required heat 

 " In making an electric lamp without a regu- 

 lator," 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 c 



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. 



Fig. 273. Edison's platinum 

 lamp. 



