Magnetic Screening
Some devices can only work properly when there is no magnetic field around. Examples are cathode ray tube, Hall effect devices, electron microscope, and high performance audio amplifiers.
To prevent magnetic fields from disturbing a sensitive device, the way is to enclose it in a magnetic material, such as soft iron.
The magnetic material guides the external field around it, in this way preventing the outside field from getting in. This is called magnetic shielding.
A very good shielding material is mu-metal. This is a nickel-iron alloy.
Mu-metal is used in electric transformers to prevent the field from affecting other devices, in computer hard drives to keep out magnetic field, and in magnetic resonance imaging.