Magnetism

Magnetism, the property of certain minerals and metals to attract pieces of iron or steel. The space where magnetic forces of attraction or repulsion manifest is called a magnetic field. Faraday showed that magnetic properties belong not only to iron and steel, but generally speaking to all bodies. However, the intensity of magnetic phenomena in a given magnetic field varies greatly and depends primarily on the substance. It is great for a small group of substances (iron and steel, nickel, cobalt, some alloys) and very small for almost all others. A piece of soft iron placed in a magnetic field itself acquires magnetic properties, or, as they say, becomes magnetized. Upon turning off the magnetic field, soft iron loses these properties and becomes demagnetized. On the contrary, steel retains the acquired magnetic properties even after the magnetic field is removed. Therefore, it is precisely from steel that the well-known horseshoe-shaped or straight permanent magnets are made. If a permanent magnet is dipped in iron filings, we find that the filings are attracted to different parts of it with unequal force. The filings are most attracted at the ends of the magnet, while in the middle they are hardly attracted at all. From this it follows that magnetic properties are mainly concentrated at the ends of the magnet, called poles. If we take a magnetic needle that can rotate freely around a vertical axis, i.e., supported for example on a point, it is known that it aligns itself in the north-south direction. The pole of the magnetic needle that points north is called the north pole, and the other the south pole. Investigating the interaction of a magnetic needle with any other magnet, we find that like poles repel, while unlike poles attract. Moreover, the force of interaction follows Coulomb's law, i.e., it decreases inversely proportional to the square of the distance between the poles. A characteristic feature of magnetism, distinguishing magnetic masses from electric ones, is that it is impossible to obtain isolated magnetic poles of one sign: both poles always appear simultaneously and cannot be separated. If a magnetized steel needle is broken in half, the two resulting pieces will be separate magnets with both poles each; the same will happen no matter how small pieces we break the needle into. This fact is fundamental to any theory of magnetism. Until now we have spoken exclusively about the magnetism of iron and steel. When studying the magnetic properties of other substances, it is found that they are not only significantly less pronounced, but also all substances are divided into two classes with respect to magnetic properties: paramagnetic and diamagnetic substances. A rod of any substance placed in a magnetic field becomes polarized in a special way, and under the influence of the forces arising from this polarization, it aligns its largest dimension either along the direction of the field (paramagnetic substances) or perpendicular to it (diamagnetic substances). The forces moving the rod arise as if from the interaction with the magnetic field of two equal in magnitude and opposite in sign magnetic masses appearing at the ends of the rod due to polarization. It turns out that paramagnetic substances in a magnetic field become magnetized such that near the exciting pole of the magnet a magnetic field of opposite sign appears, whereas in diamagnetic substances a magnetic field of the same sign appears. The impossibility of obtaining free magnetic masses historically led to the idea of the dipolarity of the basic carriers of magnetic properties, which received the name elementary magnets (Weber). In dia- and paramagnetic bodies, such elementary magnets are atoms and molecules. After the discovery of the magnetic forces of current by Ampère, an attempt was made to explain the magnetic properties of molecules by means of the constant 'molecular currents' surrounding them. From the modern point of view, each electron orbit of an atom with an electron rotating on it is such a molecular Ampèrean current. Practically, the greatest interest is what is called ferromagnetism, the phenomenon of enormous ability to become magnetized under small magnetizing fields, exhibited by iron, cobalt, and nickel, their alloys, as well as alloys of some non-ferromagnetic metals. A characteristic feature of ferromagnetic substances is that their magnetic properties, measured by the so-called 'magnetic permeability', essentially depend on the intensity of the exciting magnetic field. At the same time, the intensity of magnetization ('magnetic induction') initially increases with the intensity of the magnetizing field, but then acquires a certain constant value indicating the appearance of magnetic saturation. The ferromagnetic properties of these metals and alloys change depending on temperature and above a certain temperature specific to a given substance (the so-called Curie point) disappear completely. Most ferromagnetic bodies exhibit what is called hysteresis (see). The theory of ferromagnetism is very little developed. Ewing attempted to explain this phenomenon. He believed that in ferromagnetic substances, significant groups of elementary magnets are arranged orderly due to the interaction of their magnetic fields. An external field first deflects them elastically from this equilibrium position, and then at a certain value of the field, a whole system of magnets is overturned into a new equilibrium position. Thus, Ewing explains, among other things, the phenomenon of hysteresis. Weiss, based on theoretical considerations, came to the conclusion that in ferromagnetic bodies below the Curie point, spontaneous magnetization, i.e., magnetization arising even in the absence of an external field, must occur. This assumption could not be verified experimentally. Many facts indicate a close connection between ferromagnetism and crystal structure. Any change in the regularity of the crystal lattice, caused by foreign impurities, heat treatment, or even mechanical deformation, strongly affects the ferromagnetic properties of the substance. As for the primary carriers of magnetic properties, it was previously believed that they were the ions forming the crystal lattice. At present, it is beginning to be thought that magnetic properties are associated with the free electrons of metallic conductivity. An iron or steel rod placed inside a wire coil through which a constant electric current passes becomes magnetized and becomes an electromagnet, acquiring the property of attracting iron bodies. This circumstance is widely used in a number of physiological and electromedical instruments (Keff's hammer, an interrupter with a variable number of interruptions, tetanomotor, measuring instruments, etc.). A massive iron rod or bundle of iron wires inside the primary coil of an inductor enhances its action.