Rayleigh law
The Rayleigh law describes the change in the magnetization of a ferromagnetic material in magnetic fields less than the coercive force. It can also be used to describe the change in the electrical polarization of a ferroelectric material at low electric fields.
Ferromagnetic materials are made between white areas which are separated by the so-called Bloch walls. If now a small magnetic field applied to the material, so this would grow between districts, as the Bloch walls are shifting. Lord Rayleigh discovered this, and deduced from the linear and quadratic dependence of the magnetization ago:
Here, the magnetic initial susceptibility, indicating the initial magnetization of the material in the small box. The Rayleigh constant describes the irreversible Barkhausen jumps.