It was usual for Edison that one job would lead to another, and the above case was no exception. While trying to perfect Bell's telephone he observed a fact which he hastened to describe in his notebook: "I have just made an experiment with a diaphragm having a dull tip resting on a rapidly moving kerosene paper. The vibrations of the human voice are cleanly impressed, and there is no doubt that I shall be able to pick up and automatically reproduce any audible sound when I get to work on it." Freed, then, from the telephone, the time had come to take care of business. A cylinder, a diaphragm, a needle and other minor tools were enough to build in less than a year the Phonograph, the most original of his inventions, a device that gathered under the same principle the recording and reproduction of sound.
Edison himself was surprised by the simplicity of his invention.
Inventions after the Phonograph
Since 1877, multiple devices similar to the phonograph were developed and the so-called "talking machines" appeared. However, only the Gramophone managed to overshadow Edison's invention, which used a flat disc, unlike the Phonograph, which used a cylinder.
Gramophone
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Sources:
Henry Tomas - Thomas Alva Edison
wikipedia.com