ELECTRICITY. - About the essence of this most important subject among those that constitute our physical universe, very little is known, although knowledge of some of its manifestations in the field of our most immediate observations, or on our scale, goes back to ancient times. Already Thales (7th century B.C.) knew of the property of amber (elektron) of attracting small light bodies when it is 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 founded with numerous experiments the first electrology, which then, for about two centuries, developed considerably, remaining however still predominantly only qualitative.
I. THE FIRST THEORETICAL FOUNDATIONS
Following the Galilean scientific or experimental criterion, that first theory developed by relying on a series of fairly simple observations and on certain hypotheses suited to interpreting them.
The fundamental observations were the following: 1) By rubbing certain bodies with suitable pieces of leather or cloth, for example a piece of hard rubber with a piece of flannel, one finds that both the one and the other remain electrified.
2) By transferring, through suitable contacts, two portions of those electrifications onto two small bodies suspended from a thin silk thread, one always observes that they act upon one another at small distances with an attractive or repulsive force, when both come either from the rubbed body or from the cloth employed, and with an attractive force if one comes from the body and the other from the cloth.
3) There exist bodies in which these electrifications remain fixed where they have been produced or placed; 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 series of intermediate properties which do not concern us here.
The first hypotheses for interpreting the aforesaid fundamental phenomena were the following:
1) electrification is considered to be determined by the distribution of an electric fluid over the surface or even within the interior of bodies.
2) There exist two kinds of electric fluids, which were first called vitreous and resinous, and later positive and negative.
3) Electric fluids of the same kind repel one another; those of a different kind attract one another. A body that is not electrified, or in the neutral state, may be considered either as totally devoid of electric fluid, or as containing equivalent and therefore balancing quantities of the two fluids of contrary kind.
The adoption of the hypothesis of fluids is in evident agreement with the mechanistic ideas by which, until beyond the middle of the 19th century, an attempt was made to interpret everything by means of substances and forces acting between them.
II. COULOMB'S LAW
This notable discovery (1785) was what allowed early electrology to pass from the qualitative stage to the quantitative one and thus to constitute itself as classical electrostatics, still useful today for the resolution of countless questions. Coulomb asked himself how two electric quantities attract or repel each other as a function of their magnitudes and their distance; and with accurate experiments he answered the question by demonstrating that the sought-after electric law is quite analogous to that of Newtonian gravitation: electric forces, unlike gravitational ones which are always attractive, can be, as already said, attractive or repulsive; in any case, however, they are proportional to the product of the two quantities of e. involved, and inversely proportional to the square of their distances.
An analogous law, as is very well known, also holds for the forces observed between magnetic poles.
Another notable merit of Coulomb's law consists in this, that, by allowing it to express the force that a given distribution of e. would exert at any point of space on a quantity of e. taken as unitary, it led to the notion and the formal definition of electric field in a theory until then and for several decades still conceived as based on action at a distance; a notion that in modern electrology, through the work first of M. Faraday and then of J. C. Maxwell, assumed a role no longer formal, but absolutely real and fundamental.
III. A. VOLTA AND ELECTRIC CURRENTS
If our knowledge of electricity had remained limited to that which follows from what has been recalled so far, we would have an electrology undoubtedly interesting, from the point of view of the knowledge of an imposing element of nature, especially after B. Franklin's identification of the electricity brought into play in physical experiments with that which is produced in the clouds and causes lightning and thunderbolts. But such an electrology would be quite paltry compared with that which has developed over the last hundred and fifty years and which is still developing at a pace that shows no sign of slowing. As is well known, the determining fact was the possibility of having at one's disposal electric currents produced at first in modest proportions with piles of various types, all resulting from the famous great discovery of A. Volta, then by means of specially designed machines, called dynamos, capable of supplying them under ever more imposing conditions and proportions.
The electric current was immediately regarded as a flow of positive electricity within conductors, generally linear or filament-like. But in themselves these currents brought no greater knowledge concerning the intrinsic nature of the mysterious electric fluids. Instead, a considerable quantity of electromagnetic, electrodynamic, electrothermal, electrochemical phenomena, etc., were discovered, which, because of their perfect regularity, made it possible to define a good number of quantities of an electric nature and to measure them. From the measurement of the quantities of electricity flowing in conductors there emerged a first striking surprise: the quantities of electricity flowing at every instant in the most modest electric circuit are far greater than those brought into play by the brilliant and noisy discharges of large electrostatic machines.
IV. FARADAY'S ELECTRICAL LAWS
Among the first and fundamental effects of electric currents to be observed are the so-called electrolytic phenomena. Solutions of certain substances called electrolytes, inserted into an electric circuit, can be traversed by the electric current, that is, apparently function as conductors, but as conductors quite different from the usual metallic conductors. The passage of the electric current through such solutions is in general accompanied by the evolution or deposition of part of the elements constituting the dissolved substance and also of the solvent itself. M. Faraday investigated these phenomena thoroughly and gave their laws. Of particular interest is the one stating that the passage of the same quantity of e. always decomposes chemically equivalent quantities of electrolytes. Now this connection, this link between a quantitative chemical property of substances and the quantity of e., constitutes one of the most profound and most fruitful results of science, even if at the moment of its enunciation it was not evident to everyone. For this was the period in which the notion of chemical equivalent was giving way before the notions of atom, molecule and valence. Matter was by then almost generally considered to be constituted corpuscu-
larly, and the aforementioned connection led to the presumption
of an analogous constitution of e., which soon proved
to be of great cognitive and practical interest.
V. IONIC DISSOCIATION
But the true course of the phenomenon of electrolysis could only be clearly set forth and formulated much later, through the work of S. Arrhenius (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 e. For example, the molecule of hydrochloric acid HCl dissociates into the ions H+ and Cl-; that of sodium chloride NaCl, into the ions Na+ and Cl-. These ions, in enormous number and uniformly distributed in the solvent, manifest externally no electric field, given their equal number and the opposite signs of 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, however minimal its intensity. This fact of being able to make these ionic substances flow into one another up to two terminal electrodes, where they lose their charge and are transformed, is of extreme theoretical and practical importance; without it, the entire enormous electrolytic metallurgical industry would not exist.
But it is also very interesting and important to realize the enormity of the ionic charges. The quantity of e. corresponding to the ionization of one gram-equivalent of ion, by Faraday's law, is 96,500 coulombs positive or negative. These enormous quantities of e. are, for example, in the case of the ionization of hydrochloric acid, conferred upon 1 gram of H+ and 35.5 of Cl-, so the whole can be contained in a glass of electrolyte. We can grasp the enormity of those charges, for example, by considering that, if it were possible to separate the aforementioned 1 gr. of H+ and 35.5 gr. of Cl-, placed at a distance of 100,000 km. they would still attract each other with the force of a ton! These are perhaps the considerations that first led to the recognition of the enormity of the forces and energies that can lie hidden in a few grams of matter. It deserves to be remembered, however, that, if it is not possible for us to produce significant quantities of individual ionic substances, we are nonetheless able to produce corpuscular rays of which each corpuscle consists of an ion, in particular, for example, a positive ion of helium, He+; and they make it possible to confirm experimentally the reliability of the ionic hypothesis.
VI. THE ISOLATED ATOM OF ELECTRICITY
Up to this point, e. has been encountered in phenomena in which it could be interpreted predominantly 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; a hypothesis that does not at all exclude the possibility that it may appear in most phenomena on our scale as a fluid, on account of the extreme smallness and the enormous number of the corpuscles that must be supposed to come into play. But in all these cases e. has always presented itself united with ordinary matter, that is, either distributed microscopically over or in it, or incorporated microscopically into it, determining its ionic state.
But the interest of studying e. also in possible phenomena in which it presents itself in isolation is evident.
One of these phenomena consists of rays emanating from the cathode of vacuum tubes in which electric discharges are made to occur, rays by now very well known and called cathode rays. A long series of experiments brought to light first the corpuscular nature of such rays, 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 possibility of procuring those corpuscles was thus allowed, their various behaviors were studied in depth. In the first place it was established that, despite the absence of any ponderable material mass, they possessed a dynamic mass, that is to say that to accelerate them it was necessary to employ a force (evidently of an electric nature) and a certain expenditure of energy, which they gave back, transformed into electromagnetic radiation, when instead they lost speed following any braking of theirs.
Strictly speaking, however, the experiments just recalled did not furnish separately the two characteristics of those corpuscles, their electric charge and their dynamic mass, but only the ratio of the two, e/m, which always turned out constant for corpuscles of not excessive speed. To obtain the individual values e and m it was necessary, by means of some further experiment, to arrive at the measurement of one of them, evidently of the charge e, operating on a single corpuscle or at most on a known number of corpuscles.
This is what R. Millikan did with full success (1908), demonstrating that the minimum charge that an electrified corpuscle can assume (that is, a corpuscle united with a single atom of electricity) is 4.77·10⁻¹⁰ electrostatic units, equal to 1.59·10⁻¹⁰ coulomb, and that therefore this must be considered the charge e. From the same experiments it also emerged, brilliantly confirmed, that in any electrification of corpuscles there always intervenes a whole number of such charges. Thus the aforementioned charge e was attributed to the atom of negative e., an atom that had already been designated by the name of electron.
Following the determination of the charge e, the dynamic mass m of the electron at low speed (practically of the so-called electron at rest) turned out to be m = 8.9·10⁻²⁰ gr.
VII. THE ATOM OF POSITIVE ELECTRICITY
Until a few years ago, physics knew of no phenomenon that brought pure positive e. into play. Positive corpuscular rays, unlike cathode rays, were not made up of purely electrical corpuscles, but of ions. Thus for a time it was thought that positive e. must always be found connected with something material, and that any discharge of this 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 research on cosmic rays, Anderson first (1932) and then Blackett and G. Occhialini (1933), among numerous tracks of electrons taking part in the reactions provoked by those rays — tracks for the most part certainly attributable to ordinary negative electrons — detected some that, with equal certainty, had to be attributed to positive electrons. And so the uncertainty about the possibility of the existence of pure positive e. was resolved, and its corpuscular nature, identical to that of negative e., was brought to light.
It is perhaps not out of place to recall that several years earlier P. A. M. Dirac, on the basis of theoretical considerations on the behavior of a generic electron, had foreseen the possibility of the existence of positive electrons as well, even foretelling some of their properties.
In any case, these positive electrons, free in our terrestrial world filled with negative electrons, have an ephemeral life; when a positive electron meets a negative electron (and this inevitably happens a very short instant after its production) both vanish and instead a photon appears with energy equivalent to that possessed by them at the instant of the encounter. The inverse phenomenon has also been confirmed: a photon of sufficient energy can vanish and give rise to the production of two electrons of opposite sign.
VIII. CONSERVATION OF ELECTRICITY
The experimental fact that when e. of one kind is produced, e. of the other kind must necessarily be produced in equal quantity, together with the established practice of attributing opposite signs to them respectively: e+ and e-, and the mechanistic scientific mentality dominant until almost the end of the 19th century, led to the formulation for e. of a conservation principle analogous to the one long admitted for ponderable matter. But the brief references above to very recent findings already suggest that this principle must be subjected to a more or less profound revision (v. ENERGIA).