The law of the condenser
In order to explain how the apparatus works, Volta uses the concept of capacity. Moreover, he defines tension (T) as the tendency of electricity to escape from an electrified body. He also introduces a relation between these two quantities and the amount of charge (Q).
What’s more he finds out experimentally that capacity is inversely proportional to the distance between the two armatures.

So if one touches the lower disc of the condenser electrometer with a charged body, while the upper one is earthed, the disc is able to receive a high electrical charge, because of the considerable capacity of the apparatus, even though the tension is low.
When the charged body is removed, the capacity decreases and the tension increases substantially, as indicated by the electroscope.