Magnetic action of the current. The experience of Oersted

The possible existence of a close connection between electricity and magnetism was assumed already by the very first researchers, struck by the analogy of electrostatic and magnetostatic phenomena of attraction and repulsion. This view was so widespread that at first Cardan, and then Hilbert, considered him a prejudice and tried in every possible way to show the difference between these two phenomena. But this assumption again arose in the XVIII century with good reason, when it was established the magnetizing effect of lightning, and Franklin and Beccaria managed to achieve magnetization with the help of a discharge of the Leyden jar. Coulomb's laws, formally the same for electrostatic and magneto-static phenomena, again put forward this problem.

After the Volta battery has been able to receive electric current for a long time, attempts to detect the connection between electrical and magnetic phenomena have become more frequent and more intense. And yet, despite intensive searches, the discovery made itself wait twenty whole years. The reasons for this delay should be sought in the scientific notions prevailing at that time. All forces were understood only in the Newtonian sense, that is, as the forces that act between the material particles along the straight line that connects them. Therefore, researchers tried to find forces of this kind, creating the devices by which they hoped to detect the supposed attraction or repulsion between the magnetic pole and the electric current (or, more generally speaking, between the "galvanic fluid" and the magnetic fluid) or they tried to magnetize the steel A needle, directing a current through it.

The interaction between the galvanic and magnetic fluids was also sought by Gian Domenico Romagnosi (1761-1835) in the experiments described by him in the article of 1802 on which Guglielmo Libri (1803-1869), Pietro Confillacchi (1777-1844) and many others later referred to , Attributing Romagnosi the priority of this discovery. It is enough, however, to read this article to make sure that in Romagnosi's experiments conducted with a battery with an open circuit and a magnetic needle, there is generally no electric current, and therefore the most that he could observe is the usual electrostatic action.

When on July 21, 1820, in a very concise article on four pages (in Latin) entitled "Experimenta circa effectum conflictus electrici in acum magneticam", the Danish physicist Hans Christian Oersted (1777-1851) described a fundamental experiment on electromagnetism, That the current in the rectilinear conductor running along the meridian deflects the magnetic needle from the direction of the meridian, the interest and surprise of the scientists were great not only because the resolution that had been sought for so long was obtained, but also because the new experience as soon became clear , Pointed to the force of a non-Newtonian type.

In fact, from the experience of Oersted it was clearly visible that the force acting between the magnetic pole and the current element is directed not along the straight line that connects them, but along the normal to this straight line, that is, it was then said to be the " ". The significance of this fact was felt even then, although it was fully realized only many years later. The experience of Oersted caused the first crack in the Newtonian model of the world.

The difficulty that science has encountered can be judged, for example, by the confusion in which the Italian, French, English and German translators were translating the Latin article of Oersted into their native language. Often, after making a literal translation, which seemed to them obscure, they brought in a Latin original note.

It is indeed unclear in Oersted's article that still remains today the explanation he is trying to give to the phenomena observed by him, conditioned, in his opinion, by two oppositely directed spiral movements around the conductor of "electrical matter, respectively positive and negative."

The exceptionality of the phenomenon discovered by Oersted immediately attracted a great deal of attention from experimenters and theoreticians. Arago, returning from Geneva, where he was present at similar experiments repeated by De la Rive, spoke about them in Paris, and in September of the same year 1820 he assembled his famous installation with a vertical current conductor passing through a horizontally placed piece of cardboard sprinkled with iron Sawdust. But the circles of iron filings, which we usually notice when carrying out this experiment, he did not find. Experimenters have clearly seen these circles since Faraday put forward the theory of "magnetic curves," or "lines of force." Indeed, quite often, to see something, you must really wish for this! Arago, on the other hand, saw only that the conductor, as he expressed it, "clings to iron filings as if it were a magnet," from which he concluded that "current causes magnetism in the gland that has not been subjected to preliminary magnetization."

All the same in 1820, Biot read out two reports (October 30 and December 18), in which he reported on the results of an experimental study he conducted with Savar. Trying to discover the law that determines the dependence of the magnitude of the electromagnetic force on distance, Bio decided to use the oscillation method, which had previously been used by Coulomb. To do this, he assembled a device consisting of a thick vertical conductor located next to the magnetic arrow: when the current is turned on in the conductor, the needle begins to oscillate with a period depending on the electromagnetic force acting on the poles at different distances from the center of the arrow to the conductor with current. Measuring these distances, Bio and Savar derived a well-known law that now bears their name, which, in its first formulation, did not take into account the intensity of the current (it was then not know how to measure it).

Learning about the results of the Bio and Savar experiments, Laplace noticed that the action of the current can be viewed as the result of individual actions on the poles of an arrow of an infinite number of infinitely small elements into which a current can be divided, and concluded from this that each element of the current acts on each pole with a force , Inversely proportional to the square of the distance of this element from the pole. The fact that Laplace took part in the discussion of this problem, says Bio in his work "Precis elementaire de physique ehre-rimentale". In the writings of Laplace, as far as we know, there is no hint of such a remark, from which it can be concluded that he apparently expressed this in an oral friendly conversation with Bio himself.

In order to supplement our information on this elementary force, Bio tried, this one, to determine experimentally whether, and if it changes, how does the effect of the current element on the pole change with the angle formed by the direction of the current and the straight line connecting the middle of the element with the pole . The experiment consisted in comparing what effect the current and current parallel to it had on the same arrow, directed at an angle. From the experience of Bio by calculation, which he did not publish, but which was certainly erroneous, as shown in 1823 by F. Savary (1797-1841), determined that this force is proportional to the sine of the angle formed by the direction of the current and the straight line , Connecting the considered point with the middle of the current element. Thus, what is now called the "first elementary Laplace law" is largely a discovery of Bio.

Mario Lezzi "History of Physics"