To demonstrate the electrophorus.
Electrophorus plastic plate, aluminum plate with handle, fur, and electroscope.
An electrophorus is a device which retains its charge so that it can act as a continuing source of charge for experiments. Charge the plastic electrophorus plate negative by rubbing it with fur. Placing the aluminum plate on top of the plastic plate and grounding it charges the aluminum positive by induction. Existence of these charges can be verified using the electroscope. The charge remains on the surface of the plastic plate, so the aluminum plate can be charged by induction a number of times before the charge on the plastic leaks away.