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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?

Chris Seymour, Lota, Queensland, Australia

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This article appeared in print under the headline 鈥淩ules of attraction鈥�