Magnetic shielding redirects magnetic field lines around an object using materials with high permeability. Mumetal is the most effective shielding material, but other alloys are also used. It is used in hospitals, home theaters, and research facilities to prevent magnetic interference. Superconducting magnets are being researched for spacecraft shielding.
Magnetic shielding is a process that limits the coupling of a magnetic field between two locations. This can be done with a number of materials, including sheet metal, wire mesh, ionized gas, or plasma. The purpose is most often to prevent magnetic fields from interfering with electrical devices.
Unlike electricity, magnetic fields cannot be blocked or isolated, which requires shielding. This is explained in one of Maxwell’s equations, of the point B = 0, which means that there are no magnetic monopoles. Therefore, magnetic field lines must terminate at the opposite pole. There is no way to block these field lines; nature will find a path to bring the magnetic field lines back to an opposite pole. This means that even if a non-magnetic object, such as glass, is placed between the poles of a horseshoe magnet, the magnetic field will not change.
Instead of trying to stop these magnetic field lines, magnetic shielding redirects them around an object. This is done by surrounding the device to be shielded with a magnetic material. Magnetic permeability describes the ability of a material to be magnetised. If the material used has a higher permeability than the object inside it, the magnetic field will tend to flow along this material, avoiding the objects inside. Thus, magnetic field lines can terminate at opposite poles, but are simply redirected.
While the materials used in magnetic shielding must have a high permeability, it is important that they themselves do not develop permanent magnetisation. The most effective shielding material available is mumetal, an alloy composed of 77% nickel, 16% iron, 5% copper and 2% chromium, which is then annealed in a hydrogen atmosphere to increase its permeability. Because mumetal is extremely expensive, other alloys with similar compositions are sold for shielding purposes, usually in rolls of foil.
Magnetic shielding is often employed in hospitals, where devices such as magnetic resonance imaging (MRI) machines generate a powerful magnetic flux. Shielded rooms are constructed to prevent this equipment from interfering with surrounding instruments or meters. Similar rooms are used in electron beam exposure rooms where semiconductors are made or in research facilities that use magnetic flux.
Smaller applications of magnetic shielding are common in home theater systems. Speaker magnets can distort a cathode ray tube (CRT) television picture when placed close to the equipment, so speakers intended for this purpose are shielded. It is also used to counteract similar distortions on computer monitors.
A number of companies will build custom magnetic screens from a diagram for home or commercial applications. Shielding using superconducting magnets is being researched as a means of shielding spacecraft from cosmic radiation.
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