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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'm sure our visitors don't steal them; they must oxidize into pure
gases in 24 to 48 hours. I called Sargent Welch to get replacement bobs but
neither they nor their vendor sell only the bobs.
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.
Paul O. Johnson
Exhibit Developer
The Science Place
Dallas, Texas
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