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Subject: Re: Mu-Metal???
From: david@............. (David Josephson)
Date: Mon, 12 Jun 2000 21:40:18 -0700 (PDT)

> > I must confess, I have not a clue what this stuff is, but am 
> > intrigued.  


> It is a metal that stops magnetic fields, like shielding. It is magnetic,
> in that magnets are drawn to it, but the field does not pass through it.
> If you were to try to pick up a sting of paper clips made from Mu-Metal,
> only the first one would be picked up, the rest would not follow.
> It was discovered quite accidentally by a (retired Colonel) immigrant
> German steelworker in Pittsburgh named Otto Von Sherberg, who hurled a
> sack of kittens into an open hearth furnace on a day that they were
> making high permiability magnet iron, hence the rather unusual name. "Mu"
> is the German for "mew" and is pronounced the same way.


Ummm, no, sorry. Mu-metal is a trade name for a nickel alloy (77% Ni,
14% Fe, 5% Cu, 4% Mo), also known as Permalloy-C and several
other names as well. "Mu" refers to the Greek letter, which is used
to indicate magnetic permeability. There is some good information on
applications of this material at

High permeability alloys do *not* stop magnetic fields in the way 
that opaque things stop light. People think that they can put a sheet of
mu-metal in the "path" of a magnetic field, for instance from a magnet,
but this does not work. They are surprised to find that the magnetic field
intensity is even higher near the edges of the sheet of "magnetic shield."
If you create a tight, *seamless* housing of mu-metal, it will conduct
the ambient magnetic field around the outside so that all points on its
surface will be at (nearly) the same magnetic potential. Variations in the
external field are greatly attenuated inside such a shield. But, the 
earth's field, or that of a magnet, is effectively like DC, or static
air pressure. You have to seal a box pretty tightly to keep the static
pressure from leaking inside. Likewise, a mu-metal shield must be a 
continuous surface enclosing some volume, in order for there to be 
significant shielding effect for the volume enclosed.  It's analogous
to electrostatic shielding using highly conductive materials; the
closer you are to the shield, the closer you are to an equipotential
surface. Mu-metal is to magnetic fields as silver is (more or less)
to electrostatic fields.

I doubt that any cats were harmed in the creation of mu-metal, the
macabre story notwithstanding.

David Josephson / Josephson Engineering / San Jose CA / david@.............

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Larry Cochrane <cochrane@..............>