To shield a magnetic field you can use a "flux channel" from a good
magnetic material like steel (not Al) which is kept below magnetic saturation.
The channel must take in the flux lines and pass then to the other end
of the magnet. A steel plate below a magnet changes the flux lines
and produces local minima AND local maxima in field strength. Typically,
the edges of the steel plate would have a maxima, so the "magnetic
shield" would only work if the plate below the magnet was VERY large where
the strong magnetic field would be "channeled" to the edges of the
steel plate. Small steel plates in the area of the magnet can produce
very unexpected results and may actually concentrate the magnetic field
in undesired areas.
Building the room out of steel ( top, bottom and all sides ) will work if
the steel is kept out of magnetic saturation. Raster monitor shields
can be made with 7 sided boxes in the monitor sits back into the box.
RF shielding is the same, except copper screen can be used instead of steel.
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Woodrow W. Conover woody@acornnmr.com
Acorn NMR Voice (510) 683-8595
46560 Fremont Blvd., #418 FAX (510) 683-6784
Fremont, CA 94538-6491