Electricity

Electricity generically indicates all physical phenomena on macroscopic scale that involve one of the fundamental interactions, electromagnetic interaction, with particular reference to electrostatics. At microscopic level, these phenomena are related to the interaction between charged particles at molecular scale: protons in the nucleus of ionized atoms or molecules, and electrons. The typical macroscopic effects of such interactions are electric currents and attraction or repulsion of electrically charged bodies.

Electricity is responsible for well-known phenomena such as lightning or electrization, and is the essential element of some industrial applications such as electronics and electrical engineering through electrical signals. Having become simultaneously the most widespread means of transport for energy in electrical networks and one of the most widespread means of transport for information in telecommunications (electrical communications), electricity has become the symbol of the modern world: it lights up homes, runs factories, and makes distant peoples close.

Historical notes: from the classical age to the XVII century

Although the Greeks had already established the existence of electrical phenomena (for example, the attraction of straws by rubbing amber), only in the Renaissance there was the problem of expanding the experiences according to an increasingly clear methodological vision. After the works of G. Fracastoro, G. Cardano and G. B. Della Porta, still greatly influenced by the scholastic culture, W. Gilbert, physician of the court of Queen Elizabeth, published in 1600 the fundamental treatise De magnete, magneticisque corporibus et de magno magnete tellure physiologia nova (The new physiology of the magnet, magnetic bodies and the great magnet of the Earth), in which he described many experiences, about 600, some of which were truly original, and tried to explain the phenomena studied with a theory that although naive (all bodies would derive from only two primary elements, earth and water, and being the water hooked, only the bodies originated from it would have the property of attracting the little bodies) allowed to reject the previous theories that held the heat responsible for the attraction and that introduced hypothetical “effluvi”. After Gilbert the experimentation had a considerable development, were designed and studied particular machines to produce electrization more and more strong, typical is the electrostatic sphere of O. von Guericke built around 1650.

In the eighteenth century there is a real explosion of interest of scientists for electrical phenomena. The electrology, it can be said, was born in this century, at the end of which there were also the first attempts to theoretically systematize the huge mass of experimental results according to the mathematical-quantitative Newtonian line, which had achieved so much success in mechanics. So in the first half of the century important results were achieved, such as the distinction between an insulating body and a conducting body, due to G. T. Desaguliers, who took advantage of some works of S. Gray, and the discovery of the existence of two types of electricity, glassy and resinous, due to C. F. de Cisternay du Cisternay. F. de Cisternay du Fay. By multiplying the experiments it was possible to demonstrate that every body can be electrified, contrary to what was believed until then, and that there are not a perfect insulator and a perfect conductor.

In 1745 P. van Musschenbroek realized the first apparatus to accumulate electricity, the so called “Leyden bottle”; the intensity of the discharges obtained with this instrument allowed to make experiments that had a wide scientific echo and electricity became a fashionable subject. It was talked about in salons, shows on “electrical” phenomena were organized, and even doctors followed these strange phenomena attentively, confident in their therapeutic use. In this era B. Franklin began research on the so-called “power of the tips”, which led him to assume that electricity was an element that by rubbing was accumulated or removed from the body.

Guided by the intuition of a deep analogy between the spark and the lightning, Franklin performed on May 10, 1752 the famous experience that would lead to the realization of the lightning rod. Among the theoretical works appeared in this period, particular importance has Dell’elettricismo artificiale e naturale libri due by G. B. Beccaria, published in 1753, in which are deepened the concepts of conductor (by Beccaria called deferent) and of insulator; moreover is introduced for the first time the important concept of resistance, analyzed and studied more deeply in the following work: Elettricismo artificiale of 1772. These investigations were then continued by H. Cavendish who began systematic measurements of resistance.

In the wake of Franklin’s studies, R. Symmer and F. Aepino, although with considerable differences between them, elaborated a theory according to which electricity was due to two imponderable electric fluids, real substances that repel or attract each other according to whether or not they are of the same sign and that move more easily in some bodies than in others. This conception accounted with a certain completeness for the experiences of the time, but being of a qualitative kind, it was considered unsatisfactory by many physicists who intended to achieve a detailed quantitative analysis. C. A. Coulomb, inspired by some works of Cavendish, conducted from 1780 to 1789 a series of observations in order to establish the mathematical form of the electric force, helped by his previous studies on the resistance of materials that had led him to build the famous torsion balance. From these experiments he concluded that electrical forces are Newtonian, that is they depend on the inverse of the square of the distances between the charged bodies. The electrology seems suddenly, even if a lot remains to be done, inserted in the magnificent mathematical apparatus that has given so much prestige to Newtonian mechanics and on this road continued the studies towards a growing mathematical interpretation.

Historical notes: the XIX century

S. Poisson in the early decades of the nineteenth century extended to electricity and magnetism the concept of potential, already introduced in mechanics by Euler, and mathematically developed in the works of P. S. Laplace; G. Green brought to completion this remarkable analytical interest in a work of 1820 entitled Essay on the application of mathematical analysis to the theory of electricity and magnetism. This predominantly mathematical address did not interrupt, however, the research of experimental physicists.

The most important discovery of this period, which had so many and so wide repercussions not only on the whole of theoretical physics but also and above all on the world of technology and production, is that of the electric current that started an intense period of fruitful research that led in a short time to the foundation of electrodynamics and electrochemistry. The initiators of this research were two Italian scientists who, in those years, gave rise to a heated and controversial debate: Luigi Galvani and Alessandro Volta. The first, professor of anatomy at the University of Bologna, had the opportunity to observe that the muscles of a frog contracted when touched with a conductor in the presence of an electrostatic machine in operation.

Subsequent experiments convinced Galvani that even the atmospheric electricity was able to stimulate the frog and that the same frog was home to electricity, called animal electricity (see bioelectricity). Galvani’s researches were taken up by Volta who, after an initial favorable attitude towards animal electricity, found, like Galvani, that the phenomenon of contraction was accentuated if the metal arc formed to cause it was made of two different metals and ended up concluding (1795) that the source of electricity is not in the living organism but in the contact of two metals.

Through subsequent experiments, Volta came to amplify the effect found by building a device producing electricity became known as “Volta’s battery” (1799), which he called “electromotive apparatus” or “apparatus column”. This realization gave a significant impetus to the study of chemical and thermal effects caused by electric current. Distinguished himself in this field H. Davy, who from the decomposition of potash and soda obtained for the first time sodium and potassium; Davy also studied the behavior of conductors at different temperatures coming to the conclusion that “the conductive power of metals varies with temperature and is lower almost in the same ratio in which the temperature is higher”. These researches were then continued by G.S. Ohm who reached the law that connects the current passing in a conductor with the section and the length of the same, and defined precisely the concept of resistance.

By then electricity was becoming one of the most important chapter of physics and chemistry. There are many results of considerable interest, including: the discovery of the thermoelectric effect, due to T. J. Seebeck in 1821, which allowed the construction of another current generator (other generators will be designed by J. F. Daniell in 1836, by W. R. Grove in 1839, by G. Leclanché in 1867, by J. L. Clark in 1878, by E. Weston in 1893); the discovery of the Joule effect; the development of electrical measurements; the in-depth study of electrolysis. But the most extraordinary results of that time concern the discovery of the deep relationship between electricity and magnetism, and the consequent detection of non-Newtonian forces. From then on the development of electricity was identified with the development of electromagnetism.