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.