As every third-grader knows, Thomas Edison invented the electric lightbulb.

Or did he? It’s painful to cast aspersions on the reputation of one of America’s heroes, but Edison, who patented his bulb in 1879, merely improved on a design that British inventor Joseph Swan had patented 10 years earlier. Swan sued Edison for patent infringement, and the British courts ruled against Edison (as punishment, Edison had to make Swan a partner in his electric company). Even the U.S. Patent Office decided in 1883 that Edison’s patent was invalid, as it also duplicated the work of another American inventor.

As it happens, Swan and Edison worked from bulb designs that had been in use since the early 1800s. The general principle was, and still is, this: When electrical current flows through the bulb’s filament, the filament heats up and glows, which produces light. The inside of the bulb is a vacuum, hence oxygen-free, so the filament doesn’t get oxidized and the glow lasts a long time.

Swan used a carbonized paper filament, but the poor quality of the vacuum in the bulb caused the carbon to disintegrate rapidly, so the bulb glowed for just 13-and-a-half hours. Edison used a better vacuum pump, and after he and his posse of assistants had tested thousands of materials, he made a filament derived from bamboo that lasted up to 1,200 hours. Today’s incandescent bulbs, in which the filament is made of tungsten, last about 1,500 hours.