Usually, sources of this interference include permanent magnets, transformers, motors, solenoids, and cables. Magnetic shields provide a path around sensitive areas to deflect magnetic flux. In addition, shielding may contain magnetic flux around a component that generates flux. The ability to conduct magnetic lines of force is called permeability, and in a magnetic shield, the degree of permeability is expressed numerically. The standard or base line is free space with a rating of one, compared to shield materials which range from about 200 to 350,000.
Shields handle two basic needs.
One prevents strong field radiation from sources such as transformers, magnets, and motors. The second shields instruments and devices from magnetic fields in an environment. Three types of materials are used for magnetic shielding - high permeability, medium permeability, and high saturation. High permeability materials have a minimum permeability value of 80,000 at B-40 and a maximum of 350,000 with a saturation point of about 7,500 gauss after heat treating. Medium permeability materials are usually used with high-permeability materials and have values of 12,500 to 150,000 with a saturation point of about 15,500 gauss.