Another approach was basically a speaker connected to a carbon microphone, which compressed loosely packed carbon granules to change its resistance. They were u

Another approach was basically a speaker connected to a carbon microphone, which compressed loosely packed carbon granules to change its resistance. They were used as repeaters in long distance telephone lines as early as 1904: https://en.wikipedia.org/wiki/Carbon_microphone#Use_as_amplifier https://en.wikipedia.org/wiki/Carbon_microphone#Use_as_ampli...