AT 9:00 a.m. on October 22, 1879, Charles Batchelor, a researcher in Thomas Edison’s “invention factory” in Menlo Park, New Jersey sat down to record the results of the previous day’s work. “We made some very interesting experiments on straight carbons made from cotton thread . . . ,” he began. The results were interesting, indeed. Earlier that month, after more than a year of frustrating efforts trying to make an incandescent light with platinum wire, Edison and his colleagues had struck out in a different direction, using filaments made of carbon, instead. That had proved to be the key decision in the invention of Edison’s light bulb.

The experiments of October 21 yielded the first truly promising results for the weary lab workers. Most notably, when a filament made of ordinary thread was carbonized and hooked up to an electrical circuit, it glowed from 1:30 in the morning to 3:00 in the afternoon. After the power was increased, it shone even brighter for another hour, before finally breaking. Here was the first clear evidence that Edison and his men were on the right track, and they worked eagerly to improve and refine their design. By the end of the year, they had the technology well enough under control to make a grand public demonstration with more than 50 shining bulbs.

Little of what went into Edison’s electric light was completely new. Researchers had been trying to make incandescent lights since 1820, mostly with spirals of thin platinum wire, which was chosen for its high melting point. In 1860, the British scientist Joseph Swan patented an incandescent lamp with a carbonized paper filament, and, by the late 1870s he, too, was getting excellent results with carbonized cotton thread.

Still, there was enough novelty in Edison’s design to justify a patent, which he quickly took out. Until it expired, he dominated the American market for electric lighting, supplying about 75 percent of the nation’s bulbs, along with, in many cases, the power to light them. Swan controlled most of Britain’s electric-lighting business until his company and Edison’s British subsidiary merged in 1883. Edison rolled up 1093 patents before he was finished inventing, while Swan, who had previously invented the dry photographic plate, did not do badly, either, developing a process for making artificial fibers that remains in use in the textile industry to this day.