(b. Trassilico, Italy, 1784; d. Florence, Italy, 5 August 1835) physics.
Although Nobili received a university training, he did not immediately become a physicist. He passed the first years of his adult life as an artillery captain in Modena and Brescia. He resigned from the military to become a professor of physics at Florence, where he conducted most of his experimental and theoretical work.
Nobili was primarily interested in the electrical current. His education in the school of Ampère had taught him to view currents as phenomena that could be analyzed by supposing the existence of a central, action-at-a-distance force between their parts. Ampère had never given a precise definition of what the current was or how it was connected with the electrical fluid of Coulomb and Poisson. This imprecision troubled Nobili, and his earliest papers attempted to clarify the nature of electrical currents (1, 2).
Nobili was impressed by the existence of what seemed to be two distinct types of electrical current. Those currents that occur whenever there is a temperature gradient across a conductor he termed “thermoelectric currents”. The currents that are generated in processes involving wet conductors, as in a Voltaic apparatus, he called “hydroelectric currents”. Although both currents exerted forces on magnets and on other current-bearing conductors in the same manner, in accordance with Ampère’s law, Ampère gave no explanation of how currents originating in two seemingly distinct fashions could produce the same range of effects. Nobili resolved this dichotomy by deciding that there was actually only one type of current: thermoelectric. He believed that the currents produced with wet conductors did not result from direct chemical action, as Volta’s followers thought, but were created by the heat generated in the chemical action.
Once Nobili became convinced that all currents resulted from the release of heat, he thought that the conjunction of heat flux and electrical current was more than coincidental; he concluded that the current is a flow of heat or caloric. Nobili firmly believed in his identification of caloric flow as electrical current, and it was this belief that led him to oppose the contemporary theory of the Voltaic decomposition of water (3).
By the 1820’s and 1830’s Volta’s successors thought that Voltaic decomposition of water depended on the generation of electromotive force produced by the contact of dissimilar metals. If a strip of zinc is joined end to end to a strip of copper, and the ends of the compound strip are placed in a vessel of water (see figure 1), then the water is decomposed into oxygen and hydrogen, and the zinc is oxidized. Volta’s group held that there were two successive processes involved here. First the contact of the zinc and the copper produced a potential difference between the zinc and copper terminals of the compound strip. Since the strip was not closed, the electrical potential could not be neutralized. A state of electrostatic tension thus resulted between the ends of the strip, in which the zinc became positively electrified and the copper became negatively electrified. Since water is composed of two elements, one of which bears negative electrical fluid (oxygen) and the other positive fluid (hydrogen),
the water particles will line up between the submerged ends of the copper-zinc strip.
This configuration produces a constant stress on the water particles and induces some of them to dissociate into electronegative and electropositive components. This dissociation initiates the second stage, in which the dissociated components are attracted to the electrified zinc and copper terminals where they are held, thereby neutralizing the potential for electrical contact which occurred when the zinc and copper were joined. It is this neutralization, a quasistatic relaxation of tension in the copper-zinc strip, that is the electrical current conceptualized by Volta’s disciples.
The Voltaic explanation was rejected by Nobili and indeed by every follower of Ampère. They believed that the forces of currents were unrelated to the forces of statical electricity; the Ampèrean forces pertained uniquely to the electrical current that was thought of as an entity distinct from the statical fluid of Coulomb. Nobili believed, furthermore, that the current was a flow of caloric, not a relaxation of statical tension; and he felt called upon to explain Voltaic decomposition in Ampèrean terms. He began by criticizing the Voltaic schema.
He reasoned that the current in the strip, according to Volta, occurred as a result of the neutralizing process of oxidation; it should therefore follow, and not precede, the commencement of oxidation. But in fact the current preceded oxidation. Nobili reasoned that when the ends of the copper-zinc strip were submerged in water, a true electrical current or caloric flow was immediately engendered between the submerged ends. As Ampère had shown, the adjacent parts of any electrical current repel one another; that is why, for example, a closed conducting loop bearing a current tends to expand. Nobili suspected that this principle was operative in the Voltaic apparatus. He thought that the current, while attempting to increase in length because of the self-repulsion of its parts, exerted a powerful force at the zinc and copper terminals between which it occurs. It was this force that wrenched the water particles apart. Nobili thus concluded that the generation of current by Voltaic means was not due to the release of statical electricity by electrochemical decomposition; rather, the release was itself the result of the presence of an electrical current.
Nobili’s explanation of the Voltaic apparatus was influential in eliminating the assumption of any direct connection between statical electricity and the electrical current. His conclusion was also essential to the ultimate acceptance of Maxwellian electrodynamics and, more significantly on the Continent, to the ability of Weber and his group to concentrate their attention on the electrical forces necessary to explain the phenomena discovered by Faraday.
Works by Nobili referred to above are:
1. “Sur une nouvelle classe de phènomènes èlectrochimiques,” in Annale’s de chimie, 34 (1827), 280–297.
2. “Sur la nature des courans èlectriques,” in Bibliothèque universelle, 37 (1828), 118–144, 180–184.
3. “De la distribution et des effects des courans èlectriques dans les masses conductrices,” ibid., 49 (1835), 263–281, 416–436.
Jed Z. Buchwald