Shielding of ELF Magnetic-Dipole Fields by Ferromagnetic Cylindrical Shells

The shielding effectiveness of ferromagnetic cylindrical shells against static and extremely low-frequency ( ELF) magneticdipole fields has been measured and calculated under various conditions. The theory is specifically developed for initially non-uniform electromagnetic fields (no restriction on the dipole-shield separation distance). In addition, the theory takes into account the finite length of the shield where necessary. Satisfactory agreement between theory and experiment is obtained over the range of parameters tested. The results presented here agree in the uniform-electromagnetic-field limit with those derived previously by King for infinite cylinders.