David,

Thanks for the explanation. Your post rings true, but I didn't say anything as
I didn't have the facts at hand as you do. Several years ago, I  attempted
making a magnetically shielded enclosure to test our magnetic sensors using 1"
I.D. steel pipe 18" long with pipe caps which turned out to be completely
ineffective. When I mentioned this to the late Richard Noble of Speake & Co.,
he told me the only effective shielding to do zero field tests was to fabricate
concentric Mu metal shields which had been annealed AFTER fabrication and
separated by an inch or more. This is very expensive. The alternative is to
simply orient the sensors east-west outdoors, away from the building and road
traffic.

The Mu metal does not "stop" the magnetic flux, it conducts it around the
object within. It's curious that most people seek an analogy with nuclear and
microwave radiation (very much higher in frequency) rather than an
electrostatic field which it most resembles. I used to do shielding
effectiveness tests on very large enclosures, and more voids in the shielding
were evident with a loop antenna at 200 KHz than with an E field dipole at
microwave frequencies.

Regards,
Erich Kern
....


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