>> Magnetic shielding: Incorrect usage, methinks.

From: Nicholas Bodley (nbodley@world.std.com)
Date: Wed Mar 28 2001 - 14:44:16 PST

Woops! Sorry, I think there's a problem, here. A permeable material
allows the the force or substance under discussion to pass through
it; see a dictionary. The confusion here is between
"permeable" and "permeability", the latter being a technical term;
wood has a permeability extremely close to 1, as conventionally
defined. Plainly, wood is permeable to magnetism. Only
superconductors are not permeable to magnetic fields, as I understand

Typical ferromagnetic materials have permeabilities greater than one,
often much greater; this means that magnetism flows through them much
more easily. There are alloys that have permeabilities in the
100,000s. (See flux-gate compasses.)

The confusion is between zero permeability and unity permeability,

For commercial magnetic shielding, see the tradenames Netic and Co-
Netic; these are preferred to MuMetal, which has its own problems. I
think the former two are superseded by newer materials.

Nicholas Bodley |@| Waltham, Mass.
Please reply to nbodley@alumni.princeton.edu
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