ELETTRICITÀ. — Very little is known about the nature of this exceedingly important substance among those that constitute our physical universe, although knowledge of some of its manifestations within the sphere of our most immediate observations, or at our scale, dates back to ancient times. Thales (7th century B.C.) already knew of the property of amber (elektron) of attracting small, lightweight bodies when rubbed. Gilbert (16th century) recognized that numerous bodies possess this property, introduced the designation electric or electrified to characterize their particular state after rubbing, and, through numerous experiments, founded the first science of electricity, which then developed considerably over the course of approximately two centuries, while nevertheless remaining predominantly qualitative.
I. THE FIRST THEORETICAL FOUNDATIONS
Following the Galilean scientific or experimental method, that initial theory developed on the basis of a series of fairly simple observations and several hypotheses suited to interpreting them.The fundamental observations were the following: 1) When certain bodies are rubbed with suitable pieces of skin or cloth—for example, a piece of hard rubber with a piece of flannel—it is observed that both one and the other remain electrified.
2) When, by suitable contacts, two portions of those electrifications are transferred to two small bodies suspended from a thin silk thread, it is always observed that, at short distances, they act upon each other with an attractive force, or with a repulsive force when they originate both from the rubbed body or both from the cloth employed, and with an attractive force when one originates from the body and the other from the cloth.
3) There are bodies in which these electrifications remain fixed wherever they have been produced or placed; and others in which they move freely, tending to distribute themselves in determinate ways. The former are called insulators, the latter conductors. Between these two categories there exists a whole range of intermediate properties that are not relevant here.
The first hypotheses for interpreting the fundamental phenomena mentioned above were the following:
1) Electrification is considered to be determined by the distribution of an electric fluid over the surface or also within bodies.
2) There exist two kinds of electric fluids, initially called vitreous and resinous, and subsequently positive and negative.
3) Electric fluids of the same kind repel one another; those of different kinds attract one another. An unelectrified body, or one in a neutral state, may be considered either wholly devoid of electric fluid or as containing equivalent—and therefore mutually balancing—quantities of the two fluids of opposite kinds.
The adoption of the fluid hypothesis is evidently consistent with the mechanistic ideas by which, until after the middle of the nineteenth century, attempts were made to interpret everything through substances and forces acting between them.
II. COULOMB’S LAW
This notable discovery (1785) enabled primitive electrology to pass from the qualitative to the quantitative stage and thus to establish itself as classical electrostatics, which is still useful today for the solution of innumerable problems. Coulomb asked how two electric quantities attract or repel one another as a function of their magnitudes and distance; and through careful experiments he answered the question by demonstrating that the sought-for electrical law is closely analogous to Newton’s law of gravitation: unlike gravitational forces, which are always attractive, electrical forces may, as already stated, be attractive or repulsive; in every case, however, they are proportional to the product of the two electric quantities involved and inversely proportional to the square of the distance between them.An analogous law, as is well known, also holds for the forces found between magnetic poles.
Another notable merit of Coulomb’s law lies in the fact that, by making it possible to express the force that a given distribution of electricity would exert at any point in space upon an electric quantity taken as unity, it led to the notion and formal definition of the electric field within a theory that until then, and for several decades thereafter, had been conceived as based on action at a distance; a notion that in modern electrology, first through the work of M. Faraday and then of J. C. Maxwell, assumed a role that was no longer merely formal, but absolutely real and fundamental.
III. A. VOLTA AND ELECTRIC CURRENTS. — If our knowledge of e. had remained limited to what follows from the matters so far recalled, there would certainly be an electrology interesting from the standpoint of knowledge of an imposing element of nature, especially after B. Franklin’s identification of the e. set in motion in physical experiments with that produced in the clouds and causing flashes and lightning. But such an electrology would be very meagre compared with that developed over the past hundred and fifty years and still developing today at a pace that shows no sign of slowing. As is well known, the decisive fact was the possibility of making use of electric currents, initially produced on a modest scale by batteries of various types, all deriving from A. Volta’s celebrated great discovery, and subsequently by means of special machines, called dynamos, capable of supplying them under conditions and in quantities of ever greater magnitude.
The electric current was immediately regarded as a flow of positive e. within conductors, generally linear or filamentary. But in themselves these currents brought no greater knowledge of the intrinsic nature of the mysterious electrical fluids. Instead, a considerable number of electromagnetic, electrodynamic, electrothermal, electrochemical, and other phenomena were discovered; their perfect regularity made it possible to define and measure a good number of quantities of an electrical nature. Measurement of the quantities of e. flowing through conductors produced a first startling surprise: the quantities of e. flowing at any given moment in even the most modest electrical circuit are vastly greater than those involved in the brilliant and noisy discharges of large electrostatic machines.
IV. FARADAY’S LAWS OF ELECTRICITY
Among the first and fundamental effects observed of electric currents are the so-called electrolytic phenomena. Solutions of certain substances called electrolytes, inserted into an electric circuit, may be traversed by the electric current, that is, apparently function as conductors, though as conductors very different from the usual metallic conductors. The passage of the electric current through such solutions is generally accompanied by the release or deposition of some of the elements constituting the dissolved substance and also the solvent itself. M. Faraday investigated these phenomena thoroughly and formulated their laws. Of particular interest is the law stating that the passage of the same quantity of electricity always decomposes chemically equivalent quantities of electrolytes. Now this association, this connection between a quantitative chemical property of substances and the quantity of electricity, constitutes one of the deepest and most fruitful results of science, even though at the time of its enunciation it was not evident to everyone. In fact, this was the period when the notion of chemical equivalent was yielding to the notions of atom, molecule, and valence. Matter was by then almost universally regarded as being corpuscular in constitution, and the aforementioned association led to the presumption of an analogous constitution of electricity, which soon proved to be of great theoretical and practical interest.V. LA DISSOCIAZIONE IONICA
But the actual course of the phenomenon of electrolysis could only be clearly expounded and formulated much later, through the work of S. Arbrenna (1885) and W. Hittorf.In the electrolyte, the molecules of the dissolved substance must be considered at least partially dissociated into their ions, consisting of two fractions of the molecules themselves, bearing respectively the aforementioned atoms of positive and negative electricity. For example, the molecule of hydrochloric acid HCl is dissociated into the ions H + and Cl -; that of sodium chloride NaCl, into the ions Na + and Cl -. These ions, present in enormous numbers and uniformly distributed throughout the solvent, manifest no electric field externally, since they are equal in number and bear opposite signs in their respective electric charges. The two hosts of innumerable positive and negative ions, interpenetrating one another, move in opposite directions under the action of an electric field, even one of minimal intensity. The fact that these ionic substances are able to flow through one another toward two terminal electrodes, where they lose their charge and are transformed, is of the utmost theoretical and practical importance; without it, the entire enormous electrolytic metallurgical industry would not exist.
It is also highly interesting and important to grasp the enormity of ionic charges. The quantity of electricity corresponding to the ionization of one gram-equivalent of an ion, according to Faraday’s law, is 96,500 positive or negative coulombs. These enormous quantities of electricity are, for example, in the case of the ionization of hydrochloric acid, associated with 1 gram of H+ and 35.5 grams of Cl−; thus the whole can be contained in a glass of electrolyte. We can, for example, gain an idea of the enormity of these charges by considering that, if it were possible to separate the aforementioned 1 g of H+ and 35.5 g of Cl−, placed at a distance of 100,000 km, they would still attract each other with the force of one ton! These may perhaps be the considerations that first led to recognition of the enormity of the forces and energies that can be concealed in a few grams of matter. It should nevertheless be recalled that, although we are unable to produce appreciable quantities of individual ionic substances, we are able to produce corpuscular rays in which each corpuscle consists of an ion—specifically, for example, a positive helium ion, He+—and these make it possible to confirm experimentally the reliability of the ionic hypothesis.
VI. THE ISOLATED ATOM OF E
Thus far, e. has been encountered in phenomena in which it could be interpreted chiefly as a hypothetical positive or negative fluid; only on the basis of its behavior in electrolytic phenomena did a different hypothesis appear appropriate: namely, that it is corpuscular in nature and therefore discontinuous. This hypothesis in no way excludes the possibility that, in most phenomena on our scale, it may appear as a fluid because of the extreme smallness and enormous number of the corpuscles that must be supposed to be involved. But in all these cases e. has always presented itself united with ordinary matter, that is, either microscopically distributed over or within it, or microscopically incorporated into it, determining its ionic state.But the interest in studying e. also in possible phenomena in which it appears in isolation is evident.
One such phenomenon is constituted by rays emanating from the cathode of vacuum tubes in which electrical discharges are produced—rays now very well known and called cathode rays. A long series of experiments first brought to light the corpuscular nature of these rays and then, through the work of J. Perrin and J. J. Thomson, their negative charge and their constitution as pure e., without any material support.
Once an easy means of obtaining these corpuscles had thus been made available, their various behaviors were studied thoroughly. First of all, it was established that, despite the absence of any ponderable material mass, they possessed a dynamical mass; that is, accelerating them required the application of a force (evidently electrical in nature) and a certain expenditure of energy, which they returned, transformed into electromagnetic radiation, when instead they lost velocity as a result of any braking.
Strictly speaking, however, the experiments just mentioned did not provide separately the two characteristics of those corpuscles—their electric charge and their dynamical mass—but only the ratio of the two, e/m, which was always found to be constant for corpuscles of not excessive velocity. To obtain the individual values of e and m, it was necessary, by means of some further experiment, to measure one of them, evidently the charge e, by operating on a single corpuscle or, at most, on a known number of corpuscles.
This was accomplished with complete success by R. Millikan (1908), who demonstrated that the smallest charge that an electrified corpuscle can carry (that is, a corpuscle associated with a single atom of electricity) is 4.77 × 10⁻¹⁰ electrostatic units, equal to 1.59 × 10⁻¹⁰ coulomb, and that this must therefore be regarded as the charge e. The same experiments also provided brilliant confirmation that, in every electrification of corpuscles, an integer number of these charges is always involved. Thus the aforementioned charge e was attributed to the atom of negative e., an atom that had already been designated by the name electron.
Following the determination of the charge e, the dynamical mass m of the electron at low velocity (practically, of the so-called electron at rest) was found to be m = 8.9 × 10⁻²⁸ g.
VII. THE ATOM OF POSITIVE E
Until a few years ago, physics knew of no phenomenon involving pure positive e. The positive corpuscular rays, unlike cathode rays, were not constituted by purely electrical corpuscles, but by ions. Thus, for a certain time, it was thought that positive e. must always be connected with something material, and that the possible discharge of the latter was not to be attributed to the loss of its positive e., but rather to a neutralizing acquisition of an equal amount of negative e.But finally, in the course of investigations into cosmic rays, Anderson first (1932), and subsequently Blackett and G. Occhialini (1933), among the numerous traces of electrons involved in the reactions produced by those rays—traces for the most part certainly attributable to ordinary negative electrons—detected some which, with equal certainty, had to be attributed to positive electrons. Thus the uncertainty regarding the possibility of the existence of pure positive e. was resolved, and its corpuscular nature, identical with that of negative e., was demonstrated.
It is perhaps not out of place to recall that several years earlier P. A. M. Dirac, on the basis of theoretical considerations concerning the behavior of a generic electron, had foreseen the possibility that positive electrons might also exist, predicting some of their properties as well.
In any case, these positive electrons, free in our terrestrial world filled with negative electrons, have an ephemeral existence; when a positive electron encounters a negative electron (and this inevitably occurs a very brief instant after its production), both disappear, and instead a photon appears, with energy equivalent to that possessed by them at the instant of their encounter. The inverse phenomenon has also been confirmed: a photon of sufficient energy may, upon disappearing, give rise to the production of two electrons of opposite sign.
VIII. CONSERVATION OF E
The experimental fact that whenever e. of one kind is produced, e. of the other kind must necessarily be produced in an equal quantity,together with the established practice of assigning them respectively opposite signs: + and −, and with the mechanistic scientific mentality dominant almost until the end of the nineteenth century, led to the formulation, for e., of a principle of conservation analogous to that which had long been accepted for ponderable matter. But the brief references above to very recent findings already suggest that this principle must undergo a more or less profound revision (v. ENERGIA).