Triboelectric amplifiers

I recently asked on sci.physics.research about the principle of
triboelectric amplifiers, a topic ignored in modern texts.
I wrote:
"The tribo-electric amplifier is exemplified by Edison's
Electromotograph telegraphic relay and Chalk telephone, and as I
remember, a WW2 German loudspeaking megaphone, which may have used a dry
mineral rotor (Agate?).
In the Chalk telephone, a small dampened chalk cylinder is rotated quite
slowly. A spring leaf connected to a diaphagm is pressed quite hard on
the chalk. A voltage between the spring leaf and the chalk varies the
friction acting on the spring leaf, causing the diaphagm to move in and
out in accordance with the applied voltage. Evidently quite low voltages
and currents are effective.
A certain amount of basic information can be Googled, but I'd like to
find out more about the effect and it's principle, and I'd appreciate it
if anyone can offer any references."
Although no-one responded on the research N.G., I was eventually able to
find information at:

I share this with the group in case anyone out there is interested in
physics etc.
All the best
Ian Macmillan