In addressing the question of who invented the incandescent lamp, historians Robert Friedel and Paul Israel list <answer>22</answer> inventors of incandescent lamps prior to Joseph Swan and Thomas Edison. They conclude that Edison's version was able to outstrip the others because of a combination of three factors: an effective incandescent material, a higher vacuum than others were able to achieve (by use of the Sprengel pump) and a high resistance that made power distribution from a centralized source economically viable.
How many inventors came up with electric lamps before Thomas Edison?
22