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mist maker / electrostatic motor



Since we're talking about ion currents, I'd like to take this opportunity
to ask a related question about something that has puzzled me.

I'm sure you're all familiar with the electrostic motor often referred to
as a "whirlygig." It's a wire with sharp pointed ends bent at right angles
and looks like a lawn sprinkler. Suspended from the middle and connected
to a high voltage source (Van de Graaff or the like), it spins around like
a Hero engine.

The explanation of how it works is simple. The very strong electric field
created at the sharp points of the wire ionizes the neighboring air. These
ions are repelled by the field, Newton's 3rd law, and the thing spins in
the opposite direction of the repelled ions. Fine, but the thing works
equally well whether the high voltage applied to it is positive or
negative. I assume that molecules in the air are going to lose electrons
and be positive ions. If that's the case, they would be either repelled or
attracted to the whirlygig, depending on its polarity. So the thing ought
to spin "forward" or "backward" depending on the polarity, like Feynman's
inverse sprinkler. What am I missing this time? Wolfgang