My conjecture about heating of the air being the cause of whirligig
motion is simply based on the seeming independence of the effect on
polarity. The light mill is well understood (I even understand it)
and the "conventional" explanation is very wrong, but it is
irrelevant to the whirligig.
Why does heated air push on a pointed wire? Well, if that's where
the air is preferentially heated, where else would one expect it to
push?
I should clarify my position a bit. I do show the whirligig *en
passant* when doing a vdG demo, but I don't explain it at all. My
favorite (which I do explain) is the unstacking pie plates. I also
take lots of sparks to my knuckle which convulses me and gives the
class something to be amused about. I do fluorescent tubes, paper
streamers, fat arcs to a grounded sphere mounted on a rather easily
tipped base so that students can see macroscopic forces result, et
cetera... My favorite electrostatics demos are the demountable
Leyden jar and the simple electrophorous.
I always start with another favorite, the hoax with the Pepsi can
and the Coke can: lay an empty deep drawn aluminum Coke can on its
side on a horizontal, nonconducting surface. Bring a (negatively)
charged hard rubber rod alongside, parallel to the axis of the can.
The can will be attracted to the rod, and it will quickly accelerate.
Move the rod to keep it pulling. Next do the same thing with a Pepsi
can and a (positively) charged glass rod. Same result. In a class of
300 students someone will challenge your claim that the "difference"
is due to the brand on the can. Explain.