Even Edison needed a break. He and Batchelor had not been able to find a material that would create incandescence in a bulb for a commercially feasible time period. So he turned to other technical challenges in need of attention.
To be viable, Edison felt that the incandescent material needed to have high resistance combined with a small radiating surface. The bulb also must be capable of working in a “multiple arc” system such that each bulb could be turned on or off without interfering with other bulbs.
These were significant challenges, and the reasons others had not been able to produce a reliable incandescent system. “The crucial point was the production of a hair-like carbon filament, capable of withstanding mechanical shock, and susceptible of being maintained at a temperature of over two thousand degrees for a thousand hours or more before breaking.” Furthermore, the filament needed to be supported in a vacuum chamber “so perfectly formed” that it would withstand thousands of hours of use in which “not a particle of air should enter to disintegrate the filament.” All of this needed to be manufactured at low cost and large quantities.
With his trademark confidence, Edison acknowledged that only he, “in the enormous mass of patiently worked-out details,” could have solved the problem. With Batchelor’s help, of course.
While toggling between platinum-iridium and carbonized-paper filaments, Edison managed to improve the vacuum process enough to extend the time of incandescence. He also improved the quality of the glass bulb. On October 21, 1879, Edison had a major breakthrough. By carbonizing a piece of cotton sewing-thread bent into a loop or horseshoe form and sealing it in a vacuum bulb, Edison was able to light up the lamp to incandescent brilliance for more than forty hours, “and lo! the practical incandescent lamp was born.”
[Adapted from my book, Edison: The Inventor of the Modern World]
David J. Kent is a science traveler and the author of Lincoln: The Man Who Saved America, in Barnes and Noble stores now. His previous books include Tesla: The Wizard of Electricity (2013) and Edison: The Inventor of the Modern World (2016) and two e-books: Nikola Tesla: Renewable Energy Ahead of Its Time and Abraham Lincoln and Nikola Tesla: Connected by Fate.