Study solid dielectrics, molecules which are natural dipoles, allow to distinguish among them substance, with a rather unusual electrical properties. As a typical representative of these substances is Rochelle salt (NаКС₄N₄0₆*4N₂0), they were called ferroelectrics. It turns out, that the molecules of the ferroelectric material does not have a centre of symmetry, and the ferroelectrics possess a pronounced anisotropy of its properties.

The permittivity of ferroelectrics showed, she is fickle and in some intervals the temperature reaches high values. For example, the maximum relative dielectric permittivity of salt about sgnetway 10 000, and barium titanate (Vato₃) — about 7000.

The geometric sum of the electric moments of all the dipoles in the unit volume of the dielectric is called polarization vector P_E. The unpolarized dielectric P_E =0, and polarized (normal) dielectric vector P_E directly proportional to the vector E. It turns out, that the permittivity of the ferroelectric depends on the electric field strength in them. This means, the polarization vector for ferroelectric material is not proportional to the vector E.

Finally, the vector polarization of a ferroelectric depends not only on E, but from the earlier state of the polarizable ferroelectric. This dependence is called dielectric hysteresis (from the Greek "hysteresis" — the backlog). Periodically changing E schedule of change at P of a ferroelectric forms a closed curve, called hysteresis loop (rice. 15.25). After reducing the external field to zero at the ferroelectric residual polarization is maintained (cut 0D).

All these properties of ferroelectrics are explained by the peculiarities of their internal structure. A ferroelectric consists of spontaneously (spontaneously) polarized fields, called domains. The polarizations of the individual ferroelectric domains in the directed differently, and the overall crystal dipole moment has. If the ferroelectric add in an external electric field, the polarization vectors of the domains will rotate in the direction of this field. Partly the ferroelectric retains its polarization after removing it from the field. It turns out, that domains exist in the ferroelectric material only at specified intervals of temperatures, under which they remain ferroelectric properties. For example, sgnetway salt, these properties exist only at temperatures from -15 to +22,5°C.