When he set himself to find a convenient electric lamp, he knew that when a strong current is sent through a thin wire, that wire becomes red hot, or even white hot, and therefore gives a light. Such a light is not a glaring one, and therefore is better in one respect than an electric arc. Edison tried a great many different wires, and although many of them gave a good light, they did not last long. A platinum wire could be made white hot for some time, but in the end it always broke in some place. As the electric arc had been constructed successfully with carbon, Edison naturally tried carbon next. Charcoal was found to be too porous, and gas carbon could not be drawn into wires. It then struck Edison that if a silk thread were subjected to great heat, it was turned black. It would, so to speak, be made into carbon like charcoal, and would not require thinning down, as it would be quite thin. He tried the threads of silk, cotton, and flax, and heated them strongly in a closed vessel. They produced very fine threads of carbon, but these threads broke when they were in the least disturbed. Of all the substances which Edison tried, a bamboo fibre was found to be the best, but even that wore away after being used in a lamp for six hundred hours. Edison next invented what he called an “electric candle.” It was a short rod made of platinum powder mixed with lime. But he shortly afterwards came to the conclusion that nothing but a fine thread resembling a wire, but made of carbon, would really answer the purpose. It is strange that this conclusion was reached just about the same time by Mr. Joseph Swan in England. The two inventors were working along the same lines, and in the end they arrived at the same point. Edison obtained good results by mixing lamp black and tar, and drawing it into a fine thread. Joseph Swan, on the other hand, soaked a thread of carbon in sulphuric acid, which made it into a kind of parchment. He then blackened it by exposing it to great heat. Both inventors were very successful, and produced lamps which were in every respect satisfactory. Both men had patented their inventions, and for some time there was great rivalry between them, but in the end they decided that it was better to be friends than enemies. They formed an alliance, and their lamps were all made in the same way, and distinguished by the word “Ediswan,” which is a combination of “Edison” and “Swan.” Cotton wool was soaked in a solution of chloride of zinc. This gave a pasty substance called cellulose. A lump of this was forced by pressure through a very small hole, and so drawn out into a fine thread. The thread was then blackened by heating it in a crucible made of plumbago, and so the fibre was finished. The fibre was called a filament. [Illustration: Fig. 65.--Electric Glow lamp with Carbon Filament.]