How does a magnetic shield work?
I am not certain what kinds of magnetic shielding is available.
The simplest way to exclude the earth's magnetic field and
other fields that may be around is to apply a compensating
field - since they simply add linearly, you can cancel out
all the external fields by tuning the compensators appropriately.
The harder way is to surround your sample in a material that
expels magnetic field (like some superconductors, and I believe
some other materials as well). Usually this requires some kind
of foil that is initially placed in a folded up position, and
then spread out to a maximum volume in order to reduce the enclosed
field by spreading it out. Maybe somebody else has a better explanation
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Update: June 2012