> > 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 www.magnetic-shield.com. 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@............. __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>