Another lost amplification technology is the mechanical amplifier - basically a transducer which is mechanically coupled to a carbon button mic diaphragm - obvi

Another lost amplification technology is the mechanical amplifier - basically a transducer which is mechanically coupled to a carbon button mic diaphragm - obviously only useful in the AF range, but it was how early telephone toll circuits were amplified and extended beyond what heavy gauge copper and loading coils could do on their own.