After his 1771 memoir, Volta abandoned Latin in favour of vernacular and chose more agile forms of communication, such as short essays and scientific letters. This choice facilitated the rapid dissemination of his research and paved the way for the announcement of an instrument that would mark a turning point in the study of electricity.
In June 1775, Volta informed Joseph Priestley and other scholars of his invention of the perpetual electrophorus. With this instrument, he refuted some previous concepts of electricity and introduced an electricity generator based on a new principle, different from friction, paving the way for induction electric machines.
The electrophorus consists of an insulating resin disc and a brass metal disc with an insulating handle. After removing any moisture, the resin is electrified by rubbing. The metal disc, placed on top of it, undergoes a redistribution of the electric fluid due to contact and induction.
By touching the upper surface of the metal disc, part of the fluid is removed, leaving the disc with an excess charge. When it is lifted and the hand is brought close to it, the rebalancing of the opposing charges produces a visible spark, a clear manifestation of the electrical phenomenon.
Since the resin retains its charge even for long periods, the operation can be repeated many times without renewing the rubbing. From a single electrification, it is therefore possible to obtain almost unlimited quantities of positive electricity, a characteristic that gives the instrument the name of perpetual electrophore.
According to the current explanation, the metal disc is charged by induction: charges of the same type as the resin on the upper side and of the opposite type on the lower side. Contact with the finger removes the surface charges, leaving the disc ready for new electricity production.
