Paradoxical sheet magnets

November 27, 2007 7:07 AM

SciFilter: Why don't these sheet magnets work as expected?

I bought some sheets of magnetic material (sample 1, sample 2) to make some magnetic board games as holiday presents. The magnets seem to be behaving somewhat paradoxically.

I would expect the top of a sheet to be polarized magnetically either north or south, with the bottom of the sheet polarized oppositely. If this were true, it would enable me to make a game board out of a piece of the sheet, and make some pieces out of the rest of the sheet to stack on top of it. They would stick magnetically because the layers would be top-bottom-top-bottom, or NSNS, with north and south in closest contact.

Instead, what I'm finding is that pieces of magnet cut from the very same sheet want to stick top-bottom-bottom-top, which I interpret as NSSN -- hence the paradox. The other way, they either stick weakly or not at all. I notice that when they're stuck this way, and I slide them against each other, there seem to be magnetic "ridges", so that the magnets skip and jump in regular patterns as I slide them.

I've tried mixing pieces cut from different samples (as linked above), but the result is the same.

Obviously, the magnets aren't polarized the way I originally thought. But how are they polarized? What's going on, and how can I salvage this situation so I can still make magnetic board games for my friends and family? Cheers.

posted by rwhe to science & nature (4 comments total)

I'm guessing you just have a sheet of rubber impregnated with magnetized metallic grains, and the bits of magnetic material are more-or-less "randomly" oriented. (That is, they probably are somewhat rotated such that each bit presents the opposite end to its nearest neighbor, within the constraints of being in the rubber matrix. The ends probably more often than not point parallel to the sheet, not perpendicular to it.)

These sheets are made for affixing to metal, not other magnets, and for that "random" orientation is sufficient. Use a sheet metal base for your games, give up stacking magetized piece on top of magnetized piece, and you'll do OK.

If you need more consistent polarity or stronger magnets, you'll have to give up the flexible rubber matrix and the monetary savings of a product without any real quality control.
posted by orthogonality at 7:18 AM on November 27, 2007

Most refrigerator-ad-style sheet magnets are rows and rows of magnetic shards oriented NSNSNSNSNSNSNSNSNS. You can test this by taking two pieces and putting them together. In one orientation, they should stick really well together, and not so well in others. While it's in that orientation, pull the pieces so that they slide against each other. If you're pulling "against the grain", they'll resist movement, and if you do it just slowly enough, they will jump apart and slap back together as the N/N and N/S parts slide past each other.
posted by cmiller at 7:25 AM on November 27, 2007

Thanks, orthogonality and cmiller, even though the answer points to my having to do things a different way. I appreciate the generous gift of your time and knowledge.
posted by rwhe at 7:35 AM on November 27, 2007

The question has been answered, but I would like to request that you post a follow-up in this thread with pictures of your homemade games when they're all finished.
posted by Greg Nog at 10:03 AM on November 27, 2007