Edison's Electric Light: The Art of Invention

Carbon and the Incandescent Lamp

“Some Difficult Requirements” 67 Carbon and the Incandescent Lamp Because of its resistance and ability to withstand high temperatures, carbon was a natural choice for use in an incandescent lamp. Of course, it had to be enclosed in either an inert gas or a vacuum to prevent it from oxidizing. But most of the early serious experimenters in incandescent lighting still concentrated on carbon, and it is not surprising that Edison started out the same way. Although Edison claimed that he had “experimented more or less since 1864 with the electric light”and with incandescent carbon as early as 1869, the first specific reference to lighting experiments is by John Kruesi in a personal memo book (apparently since lost) that was presented as evidence to the U.S. Patent Office. Notations indicated that such work was done in Newark on January 5 and 6, 1877. There is no mention of the incandescing material, but since vacuum apparatus was apparently used, it was probably carbon. Further details are lacking, and in 1881 neither Kruesi nor Edison could recall anything beyond what was recorded. Carbon was certainly not unknown in electrical laboratories.At Menlo Park in the summer and fall of 1876 paper and cardboard were being carbonized in bulk quantities, and not only for use there.The carbon was also used for the production of wires, resistances, battery electrodes, and other items to be sold to the American Novelty Company of New York, recently founded by Edward Johnson, who came into Edison’s employ at Menlo Park in 1880. In 1877 more carbon was needed for Edison’s telephone transmitters. Charles Batchelor testified, and Edison confirmed, that in August or September of 1877 he cut strips from one or more carbonized sheets and brought them to incandescence in a vacuum.As Edison recalled, the carbon oxidized, a result the laboratory workers tried to prevent by coating it with molten glass.The device used was a relatively common piece of vacuum demonstration apparatus, known as a Gassiot tube or electric egg, purchased some time before and modified to fit their needs. What they had in the end was an arrangement of two brass rods supporting the incandescing material under a glass cover. Further lighting experiments in 1877 were documented by papers presented in evidence.Two, dated November 1 and December 3, indicated the use of silicon, boron, and other substances in place of carbon.Another paper, dated October 5, indicated an early understanding of the value of parallel circuits.As Edison noted in reference to his drawings of November 1, he and his researchers “tried boron, ruthenium, chromium, and the almost infusible metals for separators in my electric light devices.”Boron, he said, being of very high resistance, would work if arranged in parallel circuits, while the very-low-resistance silicon would have to be arranged in series. Edison apparently did not return actively to research on lighting until the fall of 1878, when, as he prepared for an intensified effort, the above-mentioned 1877 papers were assembled with others and copied into the newly begun series of notebooks labeled “Experimental Researches .”Interest in carbon was revived, according to Edison, at the same time. This was confirmed by Batchelor, who testified that he coated tissue paper with 68 edison’s electric light lampblack and tar, rolled it up into rods, and tested its incandescence by heating it in a vacuum. Slivers of wood and broom corn were also tried. In a discussion early in 1879 on the need for a high-resistance element, Edison remarked, according to Batchelor, on “how easy it would be to get this resistance if carbon were only stable.”Then the subject was dropped. Periodic references to the use of carbon for electric lamps appear in documents prior to Edison’s October 1879 experiments , but the most intriguing is an account by Aaron Solomon in California in a letter postmarked July 25, 1879. Solomon wrote that he had assisted a gentleman in England (presumably Joseph Swan) in an exhibition of electric lights with incandescing elements of carbon, platinum, and something said to be better than platinum. Edison’s response indicated interest only in platinum, going on to ask if Solomon knew of any sources for it in California. A brief account of Swan’s lamp as it was demonstrated before the Newcastle Chemical Society appeared in an article by B. S. Proctor that was reprinted in the July 12, 1879, issue of Scientific American. It began: At our...