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The Science Place has set up an exhibit based on the Lenz's law demo from
Sargent Welch. It consists of two 1-in. ID, 3-ft long tubes -- one
aluminum and one clear plastic, hanging from spring scales. The explorer
drops two visually identical bobs -- one magnetic and one not magnetic. --
down the tubes. Both bobs fall normally in the plastic tube, and the
non-magnetic bob falls normally in the aluminum tube. But the magnetic bob
falls very slowly in the aluminum tube, and increases the tube's weight
while it is falling. It's very mysterious and provides an excellent
vehicle for explaining magnetic induction and Lenz's law.
Our problem is that the little bobs have a tendency to disappear.
I am soliciting ideas on how to replace the bobs. They are cylindrical,
roughly 3/4-inch diameter and 2-inches long, are visually identical. If
necessary, we could use non-cylindrical bobs as long as they are visually
identical and the correct size.