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Q & A: shielding magnets

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Most recent answer: 10/22/2007
Q:
Hello, I was wondering if there is a substance or material that can be applied or added onto one side of an iron bar such that the applied side can become non-attractive to a magnet (or become significantly less attractive to a magnet). What I am trying to do here is that I want a bar (or ring) that has the property to stick to a magnet on one side, but does not on the other side, or in the case of the ring, be attractive on the inside of the ring, but not the outside. Thank you,
- David (age 22)
Philadelphia, PA
A:
With the bar, any non-magnetic substance (aluminum, plastic, etc) that you add on to one end will keep that end away from magnets and make it less easy to attract than the other end.
For the ring, you could try sticking an aluminum washer on the ring to keep magnets away from the outside, but not (if the washer hole is big enough) the inside. There are also ways to mold the shape of an object to make one part more attractive to magnets than another. For example, I believe that an iron horseshoe will be more attracted to a magnet near its gap than to one close to any other part.

Mike W.

You may need to make the nonmagnetic spacer fairly thick, depending on how non-sticky you want the nonmagnetic side to be. Magnetic fields penetrate ordinary, nonmagnetic materials, and will cause the magnetic part of your ring or bar to be attracted from a distance, but the force of attraction will be smaller the farther you get it away from the magnet. So the spacer just has to be big enough to hold the magnetic part far enough away so it won’t stick.

I saw a great picture just recently which shows how a medical MRI machine -- essentially a big magnet you get into which is used to help image parts of the body via a clever interaction with radio waves and the spins of the atomic nuclei in your body -- can attract anything ferromagnetic around it causing it to fly into the machine. A desk chair was stuck to the open side of the MRI magnet -- it has plenty of nonmagnetic materials, as well as some steel tubes. The MRI magnet was so strong and big it pulled the entire chair up to it, even with the nonmagnetic spacer materials (the cushion of the chair). The best way to prevent such sticking is just to keep chairs and anything else magnetic very far away from a big magnet like an MRI machine.

Tom


(published on 10/22/2007)

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