I have phrased similarly another question about how physicists knew that two charges exist, positive and negative. The purpose of the question is not necessarily to educate me historically. It's just that I wish to know about classical subjects without making the atomic assumption.
I know that electrons (elementary negatively charged particles) move in contrast with protons (elementary positively charged particles) because electrons have small mass and orbit the nucleus while protons are stuck in the nucleus of atoms. Roughly at least!
Charging by induction works because of the transfer of electrons, negative charge between a conductive sphere and the ground. But it could be very well explained by the transfer of either positive or negative charge. But only the later happens.
Was there any (thought) experiment to show that only negative charges happen to move/transfer?