Edison did some of his greatest work at Menlo Park. While experimenting on an underwater cable for the automatic telegraph, he found that the electrical resistance and conductivity of carbon varied accordingly to the pressure it was under. This was a major theoretical discovery, which enabled Edison to invent a "pressure relay" using carbon rather than magnets, which was the usual way to vary and balance electrical currents. In February of 1877 Edison began experiments designed to produce a pressure relay that would amplify and improve the audibility of the telephone, a device that Edison and others had studied but which Alexander Graham Bell was the first to patent, in 1876. By the end of 1877 Edison had developed the carbon-button transmitter that is still used today in telephone speakers

By the summer of 1879 Edison and Francis Upton had made enough progress on a generator that considered offering a system of electric distribution for power, not light. By October Edison and his staff had achieved encouraging results with a complex, regulator-controlled vacuum bulb with a platinum filament, but the cost of the platinum would have made the incandescent light bulb to costly. While experimenting with an insulator for the platinum wire, they discovered that, in the greatly improved vacuum they were now achieving through advances made in the vacuum pump, carbon could be maintained for a longer amount time without complicated devices. Edison found that a carbon filament provided a good light with the simultaneous high resistance required for subdivision. Steady progress ensued from the first breakthrough in mid-October until the initial demonstration for the backers of the Edison Electric Light Company on December 3.

Many of Thomas Edison's inventions including the carbon transmitter were in response to demands for new products and improvements. In 1877, he achieved his most unique discovery, the phonograph. During the summer of 1877 Edison was attempting to devise for the automatic telegraph a machine that would transcribe a signals as they were received into a form of the human voice so that they could then be delivered as telegraph messages. Some researchers had theorized that each sound, if it could be graphically recorded, would produce a distinct shape resembling short hand, or phonography, as it was known then. Edison hoped to make this concept real by employing a stylus-tipped carbon transmitter to make impressions on a strip of paraffined paper. To his amazement, the barley visible indentations generated a vague sound when the paper was pulled back beneath the stylus.

Edison's role as a machine shop operator and small manufacturer was crucial to his success as an inventor. Unlike other scientists and inventors of the time, who had limited means and lacked a support organization, Edison ran an inventive establishment. He was the antithesis of the lone inventive genius, although his deafness enforced on

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