Researching materials and inspirations for an upcoming ruler project, I discovered invar, a metal resistant to thermal expansion (changing size depending on the temperature).  Essentially steel, it has another interesting property:

“I have heard that Invar should not be roughly handled, such as dropped. Apparently this will disturb the grain structure and/or cause it to become magnetized to a small extent by the earth’s magnetic field.”

Via: Metalworking Compendium at the University of Wyoming

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