How the electrophorus functions
In Volta’s view, the absence of electrical signs after discharge comes from the balance between the positive electricity left on the mastic and the negative electricity induced on the shield. So when they are separated, there is no fresh generation of electricity on the resin or the shield (Beccaria’s electricitas vindex); there is simply an indication of two contrary electricities still existing and which can no longer reach equilibrium. After the discharge, if the experimenter holding the insulated handle puts the shield onto the resin and then lifts it off again, another strong negative charge is to be found on the resin. If the operation is repeated, it is found that the shield can continue to provide negative electricity for a long time, sometimes even for months. This is why Volta called his apparatus the perpetual electrophorus.

How the perpetual electrophorus works