Meredith Lamb writes

> For a shielded enclosure that contains a strong neodymium
> magnet (mass), would that internal magnetic field, enhance
> the overall shields ability to help attenuate the EXTERNAL
> magnetic fields influence?  To me, it would seem to help
> attenuate the external fields influence...as it would somewhat
> create a polarized shield according to the orientation of the
> internal magnet.  Any thoughts?

I don't think so. The permeability of ferromagnetic materials
is reduced when the applied field is increased, so the magnetic
"conductivity" of the shield would be reduced. Best bet to 
reduce the effect of the earth's field (DC, therefore very hard
to make an effective shield against) is to install magnets or
electromagnets to produce a mirror image of the earth's field 
at the point where  you want the field to be zero.

-- 
David Josephson / Josephson Engineering / San Jose CA / david@.............
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