Re: mist maker / electrostatic motor

[William Beaty <billb@ESKIMO.COM>]

On Wed, 17 May 2000, Leigh Palmer wrote:

> Expansion of the air near the tip may push everything in its vicinity
> away, including the tip. The effect is independent of polarity; that
> is the reason for my conjecture. What has a ramjet to do with it?

To create a continuing force, the tip would have to accelerate gas
continuously.  If a volume of gas expands in all directions away from the
tip, then the net force upon the tip would be small or zero.  A ramjet
would do what you envision: cold gas flows in from one direction, is
heated, and the hot gas flows out at a higher velocity.  But something
would have to constrain the expanding gas so that it applies a directional
force to the needle, and so it flows linearly through the hot region and
exits in a tight stream.  I think some of the current designs for a
hypersonic aircraft use an "external combustion" ramjet, where fuel is fed
along the surface of a cone, and as the mixture explodes, it pushes the
cone forwards (like shooting a wet watermelon seed from between fingers.)

If needles create "electric wind" via a thermal effect, it would have to
employ the physics of a ramjet, no?  Since the "fuel" appears continously
on the metal surface, the ramjet effect might run backwards if the spinner
was initally moving the "wrong" way before the HV was applied.


> A van de Graaff Generator is self-excited. If it is symmetrical
> (top and bottom takeoff brushes identical) it should be possible to
> produce either polarity of charge on the dome.
> I've tried initially
> biasing our vdG with a battery, but I always get the same final
> polarization direction. Should I try a Wimshurst to bias it?

The rollers also must be symmetrical, i.e. composed of identical material.
Are both of your rollers made of metal?  If so, you could initially bias
it by removing the sphere and lower enclosure, then holding a charged
balloon or rubber rod near one roller or the other before starting the
motor.

Tabletop VDGs often employ one plastic roller and one metal roller, and in
that case you'd need to swap the rollers in order to reverse the polarity.
One possible solution (which I haven't verified) :  refit your VDG with
two plastic rollers, then put a layer of any desired material on them to
create the asymmetry.  To create a "metal" roller, glue on some aluminum
foil.  Or try various kinds of adhesive tape.  In theory, only the surface
of the roller is important, although a plastic-coated metal roller might
act like a capacitor plate and give decreased output.


> I figure if anybody would know, Bill will, and everyone else will
> want to hear his answer.

Heh.  I suspected I might be getting TOO verbose recently.  :)


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William J. Beaty                                  SCIENCE HOBBYIST website
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