Electric Lamp

Lamp Diagram 1879. On 21-22 October 1879, Edison and his staff conducted their first successful experiments with a carbon-filament lamp in a vacuum. The filament was made from a piece of carbonized thread. By New Year's he was demonstrating lamps using carbonized cardboard filaments to large crowds at the Menlo Park laboratory. A year later, Edison began manufacturing commercial lamps using carbonized Japanese bamboo as filaments.

When Edison began working on electric lighting in September 1878, he made his lamps with platinum wire filaments because the metal had a high melting point. However, he discovered that oxygen attacked and weakened the platinum when it was heated. To overcome this problem he placed the metal filament in a vacuum bulb. While this improved the performance of his lamps they were still too expensive for the electrical system he was designing. Not only was platinum a very expensive metal, but it also had a low resistance to the electric current. This meant that his distribution system would need large and expensive copper-wire conductors. Unlike many of his contemporaries in the scientific and technical communities, Edison realized that Ohm's and Joule's laws required that a system of incandescent lighting use high-resistance lamps in order to reduce the size and thus cost of copper conductors. Because he had developed such a good vacuum lamp, he was able to turn to carbon, which naturally had high resistance but would burn up too rapidly in the atmosphere.

Carbon Horseshoe Lamp. The first newspaper account of his successful carbon lamp describes the "eureka" moment when he realized he could make carbon into a wire-like filament:

Sitting one night in his laboratory reflecting on some of the unfinished details, Edison began abstractedly rolling between his fingers a piece of compressed lampblack until it had become a slender thread. Happening to glance at it the idea occurred to him that it might give good results as a burner if made incandescent. A few minutes later the experiment was tried, and to the inventor's gratification, satisfactory, although not surprising results were obtained. Further experiments were made, with altered forms and composition of the substance, each experiment demonstrating that the inventor was upon the right track.

The lampblack was a common material in the laboratory because Edison used it for his telephone transmitter.

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