Invented by Volta, this instrument is the first example of an electrostatic machine based on the induction principle.
The earthed shield is placed on the plate which has previously been charged by rubbing it with a cloth. The shield is earthed by touching it with one's finger. Once the earth is removed, the shield is lifted and becomes charged by electrostatic induction. Without needing to rub the plate again, the shield can be repeatedly put on the plate and removed, thus getting quite a high charge each time. This machine could continually supply a lasting electric charge. It therefore was known as the "perpetual electrophorus".