oooooooooooo10° PB B 8888888888888888888∞8888888888888888 B0=10n B=u Bo=bou nl=anl Here u is defined as the permea bility of material. For the three kinds of magnetic substances, we have or vacuun 真空 0J0 r paramagnetic subs tance顺磁质 for diamagnetic subs tance抗磁质 Ao for ferromagnetic subs tance铁磁质B nI 0 = 0 Here  is defined as the permeability of material. For the three kinds of magnetic substances, we have           = for ferromagnetic substance for diamagnetic substance for paramagneti c substance for vacuum 0 0 0 0      真空 顺磁质 抗磁质 铁磁质 • P  B0  0 I • P  B  0 I B B nI nI = r  =  r = 