Around 1780, Volta observed that the charged shield of an electrophorus, when placed on weakly conductive materials such as marble or wood, retained electricity better than when isolated in air. He thus understands that the charge attracted to the surface of the support induces an opposite charge on the facing surface of the shield.
This observation led to the invention of the electric capacitor: two identical metal discs, superimposed and separated by a thin insulating layer. One is fixed to an insulating pedestal, the other is equipped with a handle, like the shield of the electrophorus. The insulating paint prevents direct contact between surfaces.
When one of the two discs is charged while the other is grounded, a charge of opposite sign is induced without direct passage of electricity. The opposite charges remain ‘condensed’ on the facing surfaces of the discs thanks to their mutual attraction and become observable only when the plates are separated.
By applying the principle of the capacitor to a straw electroscope, Volta created the capacitor electrometer. The rod of the electroscope supports a metal disc coated with insulating paint, on which a second disc with an insulating handle is placed. The addition of discs greatly increases the sensitivity of the instrument.
During electrification, opposite charges accumulate on the discs but remain invisible as long as they remain in contact. When the upper disc is removed, the constraint is broken, the fluid is redistributed in the electroscope and the straws diverge, making extremely weak electrifactions perceptible.
With the capacitor electrometer, Volta was able to detect the weak opposite charges produced when different metals come into contact. This discovery marked the starting point for research into contact electricity and led directly to the invention of the battery.
