It had been observed that an electroscope in a vessel at earth potential gradually lost its charge, even if very carefully insulated, due to the ionization of the air.
I differed in having horizontal electroscope rudders on the trailing edge, inboard of the movable wingtips, and which acted to provide differential drag.
But when the inducing charge is moved away, the charge is released and spreads throughout the electroscope terminal to the leaves, so the gold leaves move apart again.
He referred to the electricity so generated as metallic electricity and decided that the muscle, by contracting when touched by metal, resembled the action of an electroscope.
His intuition was that the tongue of copper wire was vibrating just like a leave of an electroscope; which means that there was an electrostatic effect.