Subject:       Re: Another mu-metal/permalloy question
To:            psn-l@..............
Date:          Tue, 13 Jun 2000 12:10:48 -0700 (PDT)
From:          david@............. (David Josephson)
Reply-to:      psn-l@..............

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@.............
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)


AUTHENTIC DIALOG (I WAS THERE) CONCERNING 
SHIELDING OF AN AIRCRAFT MAGNETIC COMPASS:

INSTRUMENT MAN: THERE IS TOO MUCH INTERFERENCE 
                FROM THE AIRCRAFT'S STEEL

MANAGER:  PUT A SOFT IRON SHIELD AROUND IT

INSTRUMENT MAN: THAT WILL EXCLUDE THE EARTH'S 
                FIELD AS WELL.

MANAGER: THEN MAKE THE SHIELD FROM ALUMINUM

HAPPY OERSTEDS

DANIE OVERBEEK
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)