Traditional electroscopes, with gold leaves, pendulums or straws, allow electricity to be estimated by observing the divergence of their moving parts. However, even when highly sensitive, these instruments did not provide sufficiently accurate measurements, prompting Volta to seek more reliable methods.
To overcome these limitations, Volta introduced the absolute electrometer, also known as the electrometric balance. The instrument consists of a balance in which one of the pans is replaced by an electrified brass plate, facing a second insulated plate or a conductive plane connected to the ground, which he called the deferent plane.
The electrification of the plate induces an opposite charge in the deferent plane, generating an attractive force. This can be measured accurately by adding weights to the other side of the scales until equilibrium is restored. In this way, Volta identifies the relationships between attractive force, quantity of electricity, size of the plate and distance between the conductors.
Volta’s results did not coincide completely with those obtained by Coulomb using a torsion balance, and for this reason the instrument did not immediately become widely used. It was not until 1845 that William Thomson, later known as Lord Kelvin, demonstrated that Volta’s conclusions were compatible with those of Coulomb and built a similar absolute electrometer, using it to measure the electromotive force of a battery.
