How does an electrophorus work to store electric charges?

An electrophorus is a plastic (insulating) plate with a metal disc on an insulating handle. The plate is rubbed to charge it. The metal disc is placed on it, earthed briefly by touching it, then lifted away.

The charged plate makes charges move in the disc without touching it directly. This is electrostatic induction. Earthing lets the unwanted charge escape, so the lifted disc keeps a charge opposite to the plate’s and can be used many times.

Magnetic induction and thermal conduction play no part.