A magnet inside a ferromagnetic box. The box material has to have a high permeability to shield an enclosed magnet from external detection. You can think of pe

A magnet inside a ferromagnetic box. The box material has to have a high permeability to shield an enclosed magnet from external detection. You can think of permeability in this instance as the ability to contain a magnetic field. As long as the shielding material doesn't saturate (too much magnetic flux), the field from an enclosed magnet can be confined within the enclosing material. No external detectable field.