Re: FUN: high-speed electrostatic air-threads

[William Beaty <billb@eskimo.com>]

On Tue, 16 Jun 1998, Karl Trappe wrote:

> Bill:  Aren't we talking about a mini-bolt of lightning?  I always assumed
> that this was a corona discharge due to electrical breakdown of the
> insulating properties of the air.  In that case a narrow stream would be
> expected because the slight warming in the vicinity of breakdown would
> increase the conductivity in the neighboring warmer air....or am I on
> another page?  Karl

Hi Karl.  I don't know.  Is warm air that conductive, or does it have a
lower breakdown threshold?  There's corona at the needle tip, but surely
not all along the several foot long air stream?  These things are not
short like sparks.  My 10KV DC supply creates "air threads" with length up
to 60cm.  I haven't tried the VDG machine yet.  Hmmm... 



By observing them with dark-adapted eyes I can see a tiny glow of corona
at the tip of the electrified needle.  However, the rest of the long
narrow stream remains totally dark.  I wouldn't be suprised if a
photomultiplier could "see" some light from ion recombination.  But my
impression is that the narrow stream is simply a jet of charged air.

My microammeter can't measure the current in one of these ion streams, the
current is lots less than 1uA.

I suddenly realized where I have seen an analogous effect: when very tiny
crystals of CuSO4 are floating on the surface of water, each crystal
releases a tendril of blue water moving downwards.  This is a laminar jet
driven by bouyancy forces.  They can become very long with no turbulence. 
Now if instead we were to turn this upside down and replace the gravity
with an e-field, then perhaps the ions streaming off of tiny bits of
corona on needle tips would move as long narrow tendrils which follow the
field? 

This still doesn't explain why the ions don't drive each other apart into
an expanding cloud.  Maybe it's a nonlinear fluid-dynamics effect, and the
ions would rather stream rapidly in response to the surrounding e-field,
instead of diffusing outwards to get away from each other.  Nonlinearities
of airflow physics would keep both from occurring simultaneously.


P S:  I bet I can write the word "PHYSICS" in the layer of mist from a
foot away.  The edges of paper seem to create a great many of these
threadlike ion streams, and the streams remain parallel.  If I build a
paper structure which has the word "PHYSICS" made from edge-on paper
sheets, then I hold this over my 10KV mist tray, the parallel rows of ion
streams should "stamp" the characters into the mist layer.  With a higher
voltage and no drafts in the room, perhaps this can be done from many feet
away?  Gotta buy more dry ice!   :)

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William J. Beaty                                  SCIENCE HOBBYIST website
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