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Re: Magnetic Shielding



On Thu, 10 Apr 1997, Ken Fox wrote:

I know that a Faraday cage will shield electric fields. Can one ( and how)
sheild against magnetic fields? I can't remember (and I don't easily find a
reference in my resources here) and was put on the spot today with that
question.

Mumetal (or other high permeability materials) will shield against
magnetic fields. However it doesn't block them in the way that conductors
block electric fields. This is because you are relying on orinetation
of domains, rather than movement of charges. If you have a gaussmeter and
a sheet of some high permeability material, you can do some experiments
with the earth's field to demonstrate:

If you orient the probe so that earth's field is normal through it, try
blocking it with the high permeability material. It doesn't work. In
fact you may see the measured field rise a little bit. (You magnetise the
material and hence increase the field). Now try placing the high
permeability sheet beside the probe and parallel to the earth's field.
The reading goes down. What is happening? Think about the magnetisation
of the material by the earth's field.... the end pointing south become a
north and the end pointing north becomes a south, so that the field that
it generates in response to the earth's filed opposes it. Take the sheet
and move it slowly and you'll see fun stuff as these ends of the induced
magnet pass by the probe...

Analysis of shielding of non-uniform fields becomes very complicated.
Suffice it to say, wrap the area in mumetal and avoid any holes or cracks
where there can be large edge effects.

****

An alternative of course is superconductors. They don't shield in the
same way as electric shielding either though. You have the Meissner
Effect repelling the field, rather than surface charges terminating it.



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| Doug Craigen |
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