555. Is there any substance that can stop magnetic fields — K, Mendenhall, MS
Magnetic fields are related to what are call magnetic flux lines. These magnetic flux lines extend unbroken from north magnetic poles to south magnetic poles. Where the flux lines are close together, the magnetic field is strong. Thus to avoid magnetic fields, you need to keep magnetic flux lines away. Because magnetic flux lines can't be broken, they can't simply be made to disappear. To "stop" a magnetic field in a particular region of space, you have to either terminate the flux lines at a magnetic pole or you have to divert the flux lines away the region that you're interested in. The first strategy has a problem: no isolated magnetic poles (so-called "magnetic monopoles") have ever been found. That means that every north pole you find has a south pole attached to it. Thus you can't simply end the flux lines with magnetic poles because for each flux line you end with a south pole, you'll start a new one with the attached north pole. But the second strategy is reasonable. There are many materials that divert magnetic flux lines. One of the most important of these is a metal called "mu metal," an alloy that's made from nickel, iron, chromium, and copper. Mu metal attracts flux lines. It draws flux lines through itself so that if you were to wrap yourself in a layer of mu metal, any magnetic flux lines that would have gone through you (and thus exposed you to magnetic fields) will go through the mu metal instead. Mu metal and similar alloys are used routinely to shield objects that can't tolerate magnetic fields.