When a magnetic field H is applied to any medium, the magnetic field strength inside the medium is not equal to H, but rather manifested as the sum of H and the magnetic polarization strength J of the magnetic medium. Due to the fact that the magnetic field intensity inside the medium is manifested by the induction of the magnetic field H through the medium, it is called the magnetic induction intensity of the medium to distinguish it from H, denoted as B: B= μ 0H+J (SI unit system), B=H+4 π M (CGS unit system) The unit of magnetic induction intensity B is T, and CGS unit is Gs (1T=104Gs). Magnetic phenomena can be vividly represented by magnetic field lines, so magnetic induction intensity B can be defined as the density of magnetic field line flux. Magnetic induction intensity B and magnetic flux density B are conceptually interchangeable.