"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:
All the best
Ian Macmillan