Used to refer to equipment (usually speakers) to describe putting a thin layer of metal (typically lead) on the inside. Why? Because speakers have big magnets inside them. And cathode ray tubes (such as found in TVs and computer monitors) work by firing electrons at the screen. The magnets from the speaker deflect the electrons, with the net result that you get funky colour effects on the screen. Cool for about five minutes, until you see your favorite pop starlet with a green face.

So, when putting speakers next to a monitor or TV you should get magnetically shielded ones. This prevents the magnetism from deflecting the electrons, so you get a nice clean image.

WARNING: If you bring magnets close to the TV screen, they can permanently damage it. This will happen if you magnetize what is called the "aperture grille" on the inside of the monitor. It's very hard to demagnetize, but you can do it if your CRT has a de-gauss button.

Magnetic shielding works on the principles of the faraday cage, except in reverse in the above example. If you have a strong conductor surrounding your dipole, any transients created by the dipole are equalized inside the conductor instead of your cathode ray tube. Magnetic shielding is also a good thing to have on your monitor if you're worried about getting TEMPESTed by someone unsavory. When Bill Clinton gave his closed-circuit grand jury deposition in the Lewinsky case, the TV set he appeared on was shielded for just this reason.

Most of the time, however, designers put magnetic shielding on a piece of equipment because they're worried about outside magnetic currents effecting their own gadget. Computers are a good example of this, and entire labs can be magnetically shielded if they are doing any sort of research with radiofrequency applications.

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