How Faraday created the theory of dielectric polarization

How is the interaction of two bodies - at a distance or through the medium? This question was posed to themselves by physicists and philosophers since the time of Newton. Newton himself practically evaded the removal of this problem, although he did not believe in the possibility of acting at a distance. Representatives of mathematical physics were inclined to recognize the action at a distance, and not so much because it is commonly believed that Newton in his studies assumed the effect of "as if" proceeding at a distance, just as for the simple reason that in the absence of satisfactory theories, The simplest model for the mathematical interpretation of phenomena. The study of electrical and magnetic phenomena again put forward this old problem. In action at a distance believed Epinus, Cavendish, Coulomb, Poisson.

Faraday tackled this issue in 1837, believing that this problem can be solved experimentally. In fact, Faraday thought, the action at a distance should be manifested only in a straight line, whereas the mediated action must be able to manifest itself along the curve; In addition, if the medium does not participate in the process of propagation of electric action, then the nature of the intermediate substance should not influence this phenomenon; If the action is mediated, then such an influence should be manifested. Guided by these ideas, he conducted numerous and witty experiments, from which it follows that the electrical action is also manifested along curved lines and that the intermediate medium significantly influences this action.

In the course of these studies, Faraday conducted his famous experiment with a wooden booth surrounded by a grounded metal grid (the "Faraday cage") within which no sign of electricity could be detected even with a very large charge on the walls, and a similar experience with the "Faraday cylinder" Which represented a more thorough and more complete experimentation with the "well" of Beccaria. These experiments of Faraday confirmed what the observers of the preceding century had already noted and what was proved, as we saw, by mathematical physics.

Carrying out experiments with spherical capacitors of the same size but with different insulating spacers, Faraday eliminated all doubts about the existence of some specific inductive ability (this term was introduced by Faraday), thus developing the research initiated by Beccaria seventy years ago.

As a result of these experiments, Faraday formulated his theory of dielectric polarization.

How to explain the influence of a dielectric in a capacitor? Avogadro in 1806 suggested that the molecules of a non-conducting body are polarized under the action of a charged conductor. Faraday, apparently, did not know this work Avogadro and was guided by two analogies: the theory of Poisson's magnetism and the theory of the electrolytic action of Grottgus.

He was struck by the similarity of a voltmeter to a capacitor: if two pieces of ice are to be applied to a piece of ice on two sides, then a condenser is obtained; if the ice is melted, a voltmeter is obtained, in which, according to the hypothesis of Grottgus, the polarized molecules are oriented in the direction of the current. But, according to Faraday, the polarization should already exist in the molecules of ice, the liquid state only allows the ions to move. Therefore, Faraday concludes, the usual electrostatic induction is "the action of adjacent particles." Particles of the body, whether it is an insulator or a conductor, are perfect conductors that are not polarized in the ordinary state, but can be polarized under the action of neighboring charged particles. A charged body placed in an insulating medium polarizes its particles layer by layer. The theory of Coulomb and Poisson magnetism, therefore, is transferred entirely to the theory of dielectrics.

At the competition announced by the Italian Scientific Society for the development of the mathematical theory of electrostatic induction, founded. On the ideas of Faraday, Ottaviano Fabrizio Mossotti (1791-1863), one of the greatest representatives of mathematical physics of the last century, whose works are now collected and published in two volumes (Pisa, 1942-1951), responded. He presented a remarkable paper, "Discussione analitica suW influenza che I'azione di un mezzo dielettrico ha sulla distributione del elettriciia alia super-jicie di piu corpi elettrici disseminati in esso" ("Analytical consideration of the effect of a dielectric medium on the distribution of electricity over the surface of electrical Bodies "), Modena, 1850

Mossotti represents a dielectric consisting of a plurality of conducting particles immersed in an insulating medium, and applies a Poisson theory of magnetism to this system. The conclusions obtained are then used to study the distribution of electricity over the surface of conductors immersed in a dielectric. The Mossotti theory was then (in 1867) applied and expanded by Clausius in his mechanical theory of heat. We shall see in the sequel how Maxwell used it. We also add that the other sources of the modern theory of dielectrics include another well-known work by Mossotti (published in Turin in 1836), in which, based on the theory of Epinus, he comes to a new theory of molecular forces and gives its analytical development.

Mario Lezzi "History of Physics"