In my business, we use rare earth magnets that are about 32 millimetres across and 8 millimetres thick. Four of them will securely hold a 1-kilogram device on a working bulldozer. But when a delivery of 1000 magnets arrives, the package has nearly no magnetic field around it. Why is that?
鈥� When a package of 1000 magnets is shipped, the magnets are arranged into cylinders. Each cylinder consists of magnets, such as those in the question, stacked with the north pole of one facing the south pole of the next, because it is impossible to group them any other way.
The pull of a stack of magnets is no stronger than that of a single one. Next, the cylindrical stacks are packed in a box in such a way that the polarities of adjacent ones are in opposite directions 鈥� so the end of the package will have north and south poles alternating like the squares on a chessboard.
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The reason for this is so that the alternating stacks largely cancel each other out in terms of their magnetic effect. It would be hazardous to ship the magnets if this were not the case.
The ends of the package will still exhibit some magnetism, but once you add in the thickness of packing material, there is only a very weak field on the outside of the package.
David Emanuel, Tulsa, Oklahoma, US
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