Have you tried the experiment under vacuum or with metal powders? I'm thinking that electric charge builds up as the electrically non-conductive grains try to move past each other, keeping them from moving much until the charge buildup is large enough to overcome air permittivity. This would release the charge and allow grain motion. If this picture is correct you should get a smaller voltage measurement in vacuum for non-conductive powder and essentially no measured voltage with the conductive powder under either vacuum or ambient pressure.
posted by Maurizio Di Mauro
September 5, 2012
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