ELETTRICITÀ

ELECTRICITY. — Little is known about the essence of this most important constituent of our physical universe, although knowledge of some of its manifestations within the scope of our most immediate observations and at our own scale dates back to ancient times. Already Thales (7th cent. B.C.) knew the property of amber (elektron) to attract light bodies when rubbed. Gilbert (16th cent.) recognized that many bodies possess this property, introduced the term electric or electrified to characterize the particular state after rubbing, and founded with numerous experiments the first electrology, which then, for about two centuries, developed considerably, remaining however predominantly still only qualitative.

I. THE FIRST THEORETICAL BASES

Following the Galilean scientific or experimental criterion, that first theory developed by relying on a series of quite simple observations and on some hypotheses capable of interpreting them.

The fundamental observations were the following:
1) Rubbing certain bodies with suitable pieces of flint and with pieces of hard rubber with a piece of flint, it is observed that both the man and the other body become electrified.
2) Bringing by suitable contacts two portions of these electrifications onto two small bodies suspended from a thin silk thread, it is always observed that they act on each other at small distances with an active, attractive or repulsive force, when both come either from the rubbed body or from the fabric used, and with an attractive force if one comes from the body and the other from the fabric.
3) There are bodies in which these electrifications remain fixed where they are produced or placed; others in which they move freely, tending to distribute themselves in certain ways. The former are called insulators, the latter conductors. Between these two categories there exists a whole series of intermediate properties which are not relevant here.

The first hypotheses to interpret the above fundamental phenomena were the following:
1) Electrification is considered to be determined by the distribution of an electric fluid on the surface or even within the bodies.
2) There exist two kinds of electric fluids, which were first called vitreous and resinous, and then positive and negative.
3) Electric fluids of the same kind repel each other; those of different kinds attract each other. A body not electrified or in the neutral state can be considered either as completely devoid of electric fluid, or as containing equivalent quantities of fluids of opposite kinds.

The adoption of the fluid hypothesis is in evident agreement with the mechanistic ideas with which, until well beyond the middle of the 19th cent., 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 enabled primitive 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 innumerable problems. Coulomb asked himself how two electric quantities attract or repel each other as a function of their magnitudes and their distance; and with careful experiments he answered the question by

showing that the electric law sought is very analogous to Newton’s law of gravitation: electric forces, unlike gravitational forces which are always attractive, can, as already stated, be attractive or repulsive; they are, however, in every case proportional to the product of the two quantities of electricity involved, and inversely proportional to the square of their distances.

A similar law, as is well known, also holds for the forces found between magnetic poles.

Another notable merit of Coulomb’s law consists in this, that by enabling it to express the force that a given distribution of electricity would exert at any point in space on a quantity of electricity taken as a unit, it led to the notion and formal definition of the electric field in a theory until then, and for several decades still, conceived as based on action at a distance; a notion which in modern electrology, first through the work of M. Faraday and then of J. C. Maxwell, assumes 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 what has been recalled so far, electrology would undoubtedly be interesting from the point of view of the knowledge of an imposing element of nature, especially after B. Franklin’s identification of electricity as that which is produced in clouds and causes lightning and thunder. This new science would, however, be a very meagre one compared to that developed in the last hundred and fifty years and which is still unfolding at a pace that shows no sign of slackening. As is well known, the determining factor was the possibility of having at one’s disposal electric currents produced at first in modest proportions by various types of devices, all stemming from the famous great discovery of A. Volta, and later by means of special machines, called dynamos, capable of supplying them in ever greater quantities and proportions.

Electric current was immediately considered as a flow of electricity, positive within conductors, generally free or filamentary. But in themselves these currents did not bring any greater knowledge about the intrinsic nature of the mysterious electric fluids. Instead, a considerable quantity of electromagnetic, electrodynamic, electrothermal, electrochemical, etc. phenomena were discovered, which, by their perfect regularity, allowed the definition of a good number of quantities of an electrical nature and their measurement. From the measurement of the quantities of electricity in conductors there resulted a first striking surprise: the quantities of electricity present at any instant in even the most modest electric circuit are vastly greater than those involved in the brilliant and noisy discharges of large electrostatic machines.

IV. FARADAY’S ELECTRICAL LAWS

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, can be traversed by electric current, i.e. they can apparently function as conductors, though as conductors very different from the usual metallic ones. The passage of electric current in such solutions is generally accompanied by the evolution or deposition of part of the elements constituting the dissolved substance or even the solvent itself. M. Faraday investigated these phenomena thoroughly and gave their laws. Of particular interest is the one which states that the passage of the same quantity of electricity always decomposes chemically equivalent quantities of electrolytes. Now this connection, this link between a quantitative chemical property of substances and the quantity of electricity, constitutes one of the deepest and most fruitful results of science, even if at the time of its enunciation it was not apparent to all. It was indeed the period in which the notion of chemical equivalent was giving way to the notions of atoms, molecules and valence. Matter was now almost generally considered to be constituted

corpuscularly, and the above connection led to the presumption of an analogous constitution of electricity, which soon proved to be of great cognitive and applicative interest.

V. IONIC DISSOCIATION

But the true course of the phenomenon of electricity could only be clearly expounded and formulated much later through the work of S. Arrhenius (1858) and W. Ostwald.

In electricity the molecules of a dissolved substance must be considered at least partially dissociated in their state, consisting of two fractions of the molecules themselves carrying respectively the aforementioned atoms of positive and negative electricity. For example, the molecule of hydrochloric acid HCl dissociated into the ions H⁺ and Cl⁻; that of sodium chloride NaCl, into the ions Na⁺ and Cl⁻. These ions, in enormous numbers and uniformly distributed in the solvent, do not manifest any external electric field due to their equal number and the opposite signs of their respective electric charges. The two arrays of innumerable positive and negative ions interpenetrating one another move in opposite directions under the action of an electric field, even if it is of minimal intensity. This fact—that these ionic substances can flow into one another up to two terminal electrons, where they lose their charge and are transformed—is of extreme technical and practical importance; without it, the entire vast 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 electricity corresponding to the ionization of one gram-equivalent of ion, by Faraday’s law, is 96,500 coulombs of positive or negative charge. 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 grams of Cl⁻; and yet all this can be contained in a glass of electrolyte. We can form an idea of the enormity of these charges, for example, by considering that if it were possible to separate the aforementioned 1 gram of H⁺ and 35 grams of Cl⁻ and place them at a distance of 100,000 km, they would still attract each other with the force of a ton! It was perhaps considerations such as these that first led to the recognition of the enormous forces and energies that can be concealed in a few grams of matter. It is worth remembering, however, that although we cannot produce significant quantities of individual ionic substances, we are able to produce corpuscular rays in which each corpuscle consists of a single ion, in particular, for example, a positive ion of H⁺; and these allow us to confirm experimentally the validity of the ionic hypothesis.

VI. THE ISOLATED ATOM OF E

Up to now the electron has been encountered in situations where it could be interpreted predominantly as a hypothetical positive or negative fluid; only on the basis of its behavior in electrolytic phenomena did it become appropriate to adopt a different hypothesis: namely, that it is of a corpuscular and therefore discontinuous nature. This hypothesis does not exclude the possibility that in most phenomena at our scale it may appear as a fluid on account of the extreme smallness and enormous number of the corpuscles that must be supposed to be involved. But in all these cases the electron is present in a quantity of ionized ordinary matter, that is, either distributed macroscopically on or in it, or incorporated microscopically in it, determining its ionic state.

But the interest is evident in studying the electron in any phenomena in which it may appear in isolation.

One such phenomenon is constituted by rays emitted from the cathode of vacuum tubes in which electric discharges are made to occur—rays now well known and called cathode rays. A long series of experiments first brought to light the corpuscular nature of such rays, and then, through the work of J. Perrin and J. J. Thomson, their negative charge and their constitution as pure electricity without any material support.

Once it became easy to obtain these corpuscles, their various behaviors were studied in depth. First of all it was observed that, despite the absence of any ponderable material mass, they possessed a dynamic mass; that is to say, when accelerated they required the application of a force (evidently of an electric nature) and a certain expenditure of energy, which they gave back in the form of electromagnetic radiation when they lost speed as a result of any braking action.

Strictly speaking, however, the experiments just recalled do not provide separately the two characteristics of these corpuscles—their electric charge and their dynamic mass—but only the ratio of the two, which always remains 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 arrive at the measurement of one of them, evidently the charge e, by operating on a single or at most a known number of corpuscles.

This is what R. Millikan succeeded in doing with full success (1908), demonstrating that the smallest charge that can be possessed by an electrified corpuscle (that is, a corpuscle united with a single atom of electricity) is 1.77 × 10⁻¹⁰ electrostatic units, equal to 1.59 × 10⁻¹⁹ coulombs, and that this must therefore be regarded as the charge. From the same experiments it was also brilliantly confirmed that in any electrification of corpuscles there always intervenes an integral number of these charges. Thus the negative atom of electricity, which had already been designated by the name of electron, was attributed the aforementioned charge e.

Following the determination of the charge e and the dynamic mass m of the electron at low velocity (practically that of the so-called electron at rest), the value m = 8.9 × 10⁻²⁸ g was obtained.

VII. THE POSITIVE ATOM OF E

Until a few years ago physics knew of no phenomenon that could involve pure positive electricity. Corpuscular rays of positive electricity, unlike cathode rays, were not constituted by purely electric corpuscles but by ions. Thus for a time it was thought that electricity must always be found connected with something material, and that any discharge of this positive electricity was not to be attributed to the loss of its positive charge but to a neutralizing acquisition of an equal amount of negative electricity.

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 that intervened in the reactions provoked by those rays—tracks for the most part certainly attributable to ordinary negative electrons—observed some that, with equal certainty, had to be attributed to positive electrons. And thus the uncertainty about the possible existence of pure positive electricity was resolved, and its corpuscular nature, identical to that of the electron, was brought to light.

It is perhaps not out of place to recall that many years earlier P. A. M. Dirac, on the basis of theoretical considerations about the behavior of a generic electron, had foreseen the possibility of the existence of positive electrons as well, even predicting some of their properties.

In any case, these free positive electrons in our terrestrial world, which is filled with negative electrons, have a very brief existence; when a positive electron meets a negative electron (and this inevitably occurs after a very short time from its production), both disappear and instead a photon of energy equivalent to that possessed by them at the moment of encounter is manifested. The inverse phenomenon is also confirmed: a photon of sufficient energy can give rise to the production of two electrons of opposite sign.

VIII. CONSERVATION OF THE ELECTRON — The experimental fact that when electrons of one kind are produced, an equal quantity of electrons of the other kind must necessarily be produced at the same time