Hello Meredith and all,

Like any magnetic material mu-metal can only carry or conduct a
limited flux density. At this point it is said to saturate. If the outside
flux is strong enough to saturate the material it appears to "leak"
through it. A given material will "leak" at different flux densities
depending on the internal structure which is affected by the
mechanical stresses that have been imposed upon it since it was
manufactured. This is why it must be heat treated to get the greatest
attenuation.

The attenuation is measured by placing a sample sleave of the material
between the coils of a set of Helmholtz coils with a pickup coil in the
center
of the test sleave. The setup we used had about 3 foot diameter coils about
2 feet apart. The coils have no magnetic material in their construction.
The coils are connected in series with their flux polarity aiding. The
pickup
coil support is made of all nonmetalic material and places the pickup coil
on the axis of the pair of Helmholtz coils. The system is calibrated with no
magnetic material near-by and no metal within the coils. the exciting AC
current (often 60 Hz) is adjusted to produce some reference level output
from
the pickup coil. The test sleave is then placed over the pickup coil with
the
pickup coil centered in the length of the sleave. The sleave length should
be
about 20 times the length of the pickup coil. Our pickup coil was about 1/2
inch long and about 1 1/2 inches in diameter.It is wound with many turns
of fine wire. Any improvement in attenuation from making the sleave longer
or closing the ends can't be measured. The radial position of the coil
within
the sleave has little effect on the accuracy.

The attenuation is expressed as a  linear ratio although I have seen dB used
in some papers I have read. I think a direct ratio is easier to understand
in this
application.

I thought this might help the understanding of how shields work and are
tested.
_______________________________

                     Al  Allworth

On the Beautiful Southern Oregon Coast

________________________________
----- Original Message -----
From: "meredith lamb" 
To: 
Sent: Tuesday, June 13, 2000 11:22 AM
Subject: Another mu-metal/permalloy question


> Hi all,
>
> Thanks for all the notes and tips on magnetic shielding.
>
> I don't have the experience to really delve into the subject
> at all.....but I do have another question, that kind of makes
> me wonder alot.
>
> 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?
>
> Thanks, Meredith Lamb
>
>
>
>
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