THE INCANDESCENT LAMP 195 



when the general excitement over my invention and exhibi- 

 tion of the phonograph out at old Menlo Park frustrated 

 serious or continuous work for a time, in any other direction. 

 In fact, my health gave way under the strain, and in July I 

 broke awa}^ for a w^estern trip as far as California. 



Of course my mind was turning the subject over, and 

 when I got back in August we immediately went at it again. 

 Around October and November Batchelor made a great num- 

 ber of paper carbons, at least 50, from tissue and other kinds 

 of paper, coated over their surface vnih a mixture of lamp- 

 black and tar, rolled up into the fine long form of a knitting 

 needle, and then carbonized. These w^e put into circuit and 

 brought up to incandescence in vacuo; although they would 

 last but an hour or two. We tried a great many experiments 

 with paper carbons, wood carbons and some made from car- 

 bonized broom corn. What we desired at that date, and had 

 settled our minds upon as the only possible solution of the 

 subdivision of the electric light, was that the lamps must have a 

 high resistance and small radiating surface. About Decem- 

 ber, 1878, I engaged as my mathematician Mr. Francis R. 

 Upton, who had lately studied under Helmholtz, in Germany, 

 and he helped me greatly in calculations of the multiple arc 

 problem. Our figures proved that the lamp must have at 

 least 100 ohms resistance to compete successfully with gas; 

 for if the lamps were of low resistance the cost of the copper 

 main conductors would be so great as to render the system 

 uneconomical and commercially impracticable. In this direc- 

 tion we tried platinum also ; and when working on incandescent 

 platinum we had procured a Sprengel mercury pump and had 

 ascertained that we could thus get exceedingly high vacua. 

 It occurred to me that perhaps a filament of carbon could 

 be made to stand in the sealed glass vessels or bulbs, which 

 we were using, exhausted to a high vacuum. Separate lamps 

 were made in this way independent of the air pump, and in 

 October, 1879, we made lamps of paper carbon, and with car- 

 bons of common sewing thread, placed in a receiver or bulb 

 made entirely of glass, mth the leading-in wires sealed in b}^ 

 fusion. The whole thing was exhausted by the Sprengel 

 pump to nearly one millionth of an atmosphere. These fila- 



