In September 1879, he experimented with thin filaments of carbon, and by the end of the year he had a practical lamp. This meant that most of the energy would be used in the light bulb and not in the distribution system, a critical factor that distinguished Edison s invention from all others. Edison got his carbon filaments by baking vegetable fibers, which he reasoned would leave the carbon atoms strongly linked together. Thread worked, cardboard was better, and even better was bamboo. He also made improvements in other parts of the lamp.
Notes are also valuable in patent cases and, eventually, to historians.
In the case below are several of Edison's notebooks. Notice the tip, which shows where the lamp was sealed at the point where the vacuum pump was attached. Also, a screw base has been added, and the glass is pressed against the lead-in wires, making a simpler seal. By early December 1879, Edison was convinced that he had developed a practical carbon-filament incandescent lamp. As soon as he could produce enough of them, he strung them around his Menlo Park compound for others to see.