Bell's Variable Resistance Microphones :
Bell from the beginning knew that he was going to need an
external power source to drive his telephone system long distances.
So one of the first experiments after receiving his patent
was designing a variable resistance microphone.
He used several variations and a whole range of resistive materials
and tested their suitability for microphone work.
This was not his strong suit. Tedious experiments were
hard for him to-do.
In the end others (Edison) solved Bells technical
problems using a carbon button resistive element.
In the beginning however Bells started with an acid cup
and a tuning fork arrangement as the sound generating device.
Its depth in the acid cup determined the current flowing
thru the transmitter. So as the prong of the fork varies up and down
the resistance varies and therefore the current from the battery which
then passes through the receiver varies.
The receiver which is on the right of the drawing is one
of Bells reed receivers tuned to the frequency of the tuning fork