The relationship between the magnetic field (H) and magnetic induction (B) in a substance is characterized by a physical quantity called magnetic permeability . Absolute magnetic medium permeability is the ratio of B to H. According to the International System of Units, it is measured in units called 1 Henry per meter.
Its numerical value is expressed by the ratio of its value to the magnetic permeability of vacuum and is denoted by μ. This value is called relative magnetic permeability (or simply magnetic permeability) of the medium. As a relative value, it does not have a unit of measure.
Therefore, the relative magnetic permeability μ is a value that shows how many times the field induction of a given medium is less (or more) than the vacuum magnetic field induction.
When a substance is exposed to an external magnetic field, it becomes magnetized. How does this happen? According to Ampere’s hypothesis, microscopic electric currents constantly circulate in each substance, caused by the movement of electrons in their orbits and the presence of their own magnetic moment. Under ordinary conditions, this motion is disordered, and the fields “cancel” (compensate) each other. When the body is placed in an external field, the currents are ordered, and the body becomes magnetized (i.e., having its own field).
The magnetic permeability of all substances is different. Based on its size, substances are divided into three large groups.
In diamagnets, the magnetic permeability μ is slightly less than unity. For example, for bismuth μ = 0.9998. Diamagnets include zinc, lead, quartz, rock salt, copper, glass, hydrogen, benzene, water.
The magnetic permeability of paramagnets is slightly larger than unity (for aluminum, µ = 1.000023). Examples of paramagnets are nickel, oxygen, tungsten, ebonite, platinum, nitrogen, air.
Finally, the third group includes a number of substances (mainly metals and alloys), whose magnetic permeability significantly (by several orders of magnitude) exceeds unity. These substances are ferromagnets. This mainly includes nickel, iron, cobalt and their alloys. For steel, µ = 8 ∙ 10 ^ 3, for an alloy of nickel with iron, µ = 2.5 ∙ 10 ^ 5. Ferromagnets have properties that distinguish them from other substances. First, they have residual magnetism. Secondly, their magnetic permeability is dependent on the magnitude of the induction of the external field. Thirdly, for each of them there is a certain temperature threshold, called the Curie point , at which it loses its ferromagnetic properties and becomes paramagnet. For nickel, the Curie point is 360 ° C, for iron - 770 ° C.
The properties of ferromagnets are determined not only by magnetic permeability, but also by the quantity I, called the magnetization of this substance. This is a complex nonlinear function of magnetic induction; the increase in magnetization is described by a line called the magnetization curve . At the same time, having reached a certain point, the magnetization practically ceases to grow ( magnetic saturation sets in ). The lag of the magnetization of a ferromagnet from the growing value of the induction of an external field is called magnetic hysteresis . Moreover, there is a dependence of the magnetic characteristics of a ferromagnet not only on its current state, but also on its previous magnetization. The graphic image of the curve of this dependence is called the hysteresis loop .
Due to its properties, ferromagnets are universally used in technology. They are used in the rotors of generators and electric motors, in the manufacture of transformer cores and electromagnetic relays, in the production of parts of electronic computers. The magnetic properties of ferromagnets are used in tape recorders, telephones, magnetic tapes and other carriers.