Stirring up a magnetic paradox

Abstract

A commerical magnetic stirrer provides a convenient rotating horseshoe magnet for several well-known demonstrations of induced eddy currents and Lenz’s law such as the spinning of a nonferromagnetic metal top or coin and the operation of a speedometer. A new demonstration is paradoxical: A steel ball rolls in a circle either in the same direction as the rotation of the stirrer magnet or in the opposite direction. That an aluminum cone or rivet also rolls in the opposite direction shows that the latter effect is due to induced currents, but this seems to violate Lenz’s law.