The invention that Edison is most remembered for is, by far, the electric light bulb. At the age of thirty-one, he decided to focus his energies–and the manpower of the Menlo Park facility–toward creating an electric light system. He began work in the fall of 1878, after returning from a vacation with the physicist George Barker. Barker had encouraged Edison to work on creating an electrical system and discussed ideas with him about it. Edison recognized the staggering potential of an electrical light system and decided to focus on creating one, having just finished work on the phonograph.

At this time, gas lamps lighted most American cities. Other inventors had already done some pioneering work in the electrical light field, especially Humphrey Davy in 1802 (who first produced "incandescence," an electric current flowing through wire) and the Englishman Joseph Swan in 1860 (who produced many experimental incandescent lamps). But no one had been able to completely solve the practical problems of creating an effective and reliable lamp.

From October 1878 until New Year's Day, 1880, Edison developed the components for a lighting system. His experience with telegraph technology assisted him as he tried to envision a system of relays and circuit breakers that would be necessary to making a lamp work. The main problems were locating the proper filament for the incandescent spiral and constructing a lamp that had enough pressure to contain the filament. He perfected new vacuum techniques for the latter problem and rejected the spiral filament in favor of a filament of carbonized thread.

When the lamp with carbonized thread lasted for forty-five hours the staff at Menlo Park realized that they had had a breakthrough. Edison claimed, "none of us could go to bed, and there was no sleep for any of us for forty hours." In November 1879 they tried using carbonized cardboard, and soon they had created an experimental bulb that was superior to any of the others they had tested.

On New Year's Eve, the Menlo Park facility was swarmed by the press, on- lookers, and Edison's financiers, all eager to see the new invention. Edison enthralled the visitors by lighting forty light bulbs all at once, then switching them on and off. The real work, however, consisted in creating a system that was inexpensive and practical: true innovation lay in lighting on a much larger scale, which required generator systems and safety fuse technology.

To develop this system, Edison depended in part upon existing gas lighting systems. He copied the distribution system of gas lighting, which was dependent upon a central power source. During the early 1880s, he set about the complicated research work of developing a system that would light many lamps all at once, without causing electrical fires or damage. This required painstaking work on everything from wire insulators to junction boxes and underground wire mains. His first generators, the "long-waisted Mary Ann" and the "jumbo" weighed hundreds of pounds and produced, in modern terms, only about 100 kilowatts.

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