When ferromagnetic material samples are magnetized in an alternating magnetic field, for the same frequency, Change the size of the alternating magnetic field (Amplitude), different dynamic hysteresis loops can be obtained, and the connection between the vertices of these dynamic hysteresis loops is called the dynamic magnetization curve. At low frequencies, the AC magnetic hysteresis loop is similar to the true current hysteresis loop reached in the previous section. However, as the frequency increases, the core of the transistor loses and the hysteresis loop widens, which will be different from the DC hysteresis loop. Therefore, there is a difference between the dynamic magnetization curve and the static magnetization curve. When the amplitude of the AC magnetic field is very small When, its magnetic induction strength

The ratio of the change to the change in magnetic field strength is called the AC initial permeability, which is also simplified to Ui. In fact, the true magnetic permeability of a material can only be measured on a uniformly magnetized closed core (such as a circular core). Therefore, the initial magnetic permeability of a material is usually measured using a standard test ring. After evenly winding n turns of the coil, the inductance value L of the coil is measured, and then calculated using the following formula:

In the formula, the unit of inductance L is H (Tingli), the outer diameter D, inner diameter d, and height h of the annular magnetic core are all measured in meters.