How does Edison's phonograph help disprove the theory of necessity as the mother of invention?
Shortly after inventing the phonograph, Edison wrote an article in which he described the potential uses of his invention. These uses included, according to Diamond:
recording the last words of dying people, recording books for blind people to hear, announcing clock time, and teaching spelling. Reproduction of music was not high on Edison's list of priorities...
Edison was frustrated, because the phonograph was not marketable in any of these uses. In fact, he was opposed to its use as a music playback device, and it was other forward-thinking businessmen who decided to put it to that use.
Diamond uses this story, along with that of Nikolaus Otto, who invented the gas engine, and James Watt, who developed the steam engine (from Thomas Newcomen's design) to pump water out of mines, to show that the relationship between necessity and invention can often be reversed. It was not need that led Edison to create the phonograph. Indeed, he didn't anticipate what uses it would be have. It also tends to downplay, he argues, the role of the "heroic genius" in advancing society's technological development.
Source: Jared Diamond, Guns, Germs, and Steel:The Fates of Human Societies (New York: W.W. Norton, 1999), 243-244.