Re: Magnetic Shielding (fwd)

Date: Mon Feb 03 2003 - 18:10:05 PST

Consider the field lines from a magnet. They form a nice dipole pattern.
Coming out of the north pole and curling around to the south.

Orient the magnet with north pole up.

Place a steel bolt above the magnet. The magnetic field from the magnet aligns
some of the iron atoms in the bolt. The bolt is attracted to the magnet and the
bolt itself becomes a magnet. The magnetic field of the bolt now adds to the
magnetic field of the original magnet, and the field pattern changes.

Now insert a planar sheet of soft iron between the magnet and the bolt.
The field lines from the magnet orient the atoms in the sheet of iron.
The combined field of the magnet and the iron is nearly zero at the bolt.

Follow a field line it enters the iron, it turns so that it is parallel to the
face of the iron and exits again on the magnet side. The field line no longer
penetrates the iron to reach the bolt.

Indeed the iron plate is attracted to the magnet. But the combined field cancels
on the bolt side.
Paul Doherty
Senior Scientist

> ---------- Forwarded message ----------
> Date: Sun, 2 Feb 2003 14:17:22 +1100
> From: Kate McGilp <>
> To:
> Subject: Magnetic Shielding
> Ok, here's the question....
> Obviously materials with low reluctance are great for magnetic shielding because they
easily redirect the flux from one pole back to the other pole without apparently
the object being shielded.
> What I don't understand is why doesn't it affect the shielded material?
> If the shielding medium has a low reluctance it's magnetic domains line up along the
lines and in doing so would become a magnet themselves??? And then wouldn't they
just as
easily affect the material that it is meant to be shielding?? Is this a bad

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