Modern magnetic research shows that all magnetic phenomena originate from electric currents, and a unit of microcurrent is called a magnetic dipole. The maximum torque generated by each unit of external magnetic field in vacuum on a magnetic dipole is the magnetic dipole moment Pm, and the vector sum of magnetic dipole moments per unit volume in the material is the magnetic polarization intensity J, with SI units in T (Tesla). The vector sum of magnetic moments per unit volume in the material is the magnetization M, and the magnetic moment is Pm/ μ 0, its SI unit is A/m (amperes per meter). Therefore, the relationship between M and J is: J= μ 0M， μ 0 is the vacuum magnetic permeability, in the SI system of units, μ 0＝4π × 10-7H/m (Heng/m).