Experimenters had been trying for forty years to create incandescent electric lamps when Edison took up the problem in September 1878. The basic challenge was to bring a piece of matter to a temperature high enough to emit useful light without it melting, oxidizing (burning), or consuming an inordinate amount of electric power. Drawing on his understanding of basic electrical laws, Edison determined that his lamps should have a high resistance of about 100 ohms to minimize both the current to be generated and the amount of expensive copper needed in conductors to carry the power. By placing his lamps in parallel circuits each one could be turned on and off without affecting others up or down the line.
Edison initially designed lamps with wire filaments of platinum because that rare metal has a high melting point. However, systematic basic research into platinum’s properties showed that it absorbed air into microscopic pores when heated, weakening the metal and lowering its melting point. By March 1879, he was isolating the filament in a vacuum bulb. Using a vacuum improved the lamps’ performance and durability, but platinum lamps were still too expensive to make and operate also had a low resistance to the electric current.
Edison’s improved vacuum technology allowed him to consider carbon as an alternative. Carbon had high resistance but would burn readily in the presence of air. The improved vacuum bulb would preserve it from air. The first newspaper account of his successful carbon lamp described the "eureka" moment when Edison realized he could make carbon into a wire-like filament by using lampblack, the same material in his carbon-button telephone transmitter. On 21–22 October 1879, he and his staff made their first successful experiments with a filament of carbonized cotton thread in a vacuum: the carbon-filament incandescent lamp. He filed the fundamental patent application just a few weeks later for the carbon-filament incandescent lamp.
After experiments with a wide variety of carbonizable materials, including the Bristol board used for New Year’s demonstrations, experiments focused on grasses and canes (such as hemp, palmetto, and bamboo) whose long, uniform fibers made sturdier and longer-lasting filaments. Bamboo turned out to be best for the commercial lamp and Edison opened his first lamp factory at Menlo Park in October 1880.
