Szczepan Bialek wrote:
"tom" wrote
t...
On 7/23/2010 4:23 PM, Jim Lux wrote:
If the ball is wet, especially with distinct droplets, then you can get
corona forming much earlier. The electrostatic forces tend to make the
droplets fly off.

Non-rhetorical question(s).

I must be missing something then. Why don't I see corona on the tips of
leaves at the tops of my trees? Trees are pretty conductive when hundreds
of kilovolts are involved.

You have missed that: "Physically, St. Elmo's fire is a bright blue or
violet glow, appearing like fire in some circumstances, from tall, sharply
pointed structures such as lightning rods, masts, spires and chimneys, and
on aircraft wings. St. Elmo's fire can also appear on leaves, grass, and
even at the tips of cattle horns.[3] Often accompanying the glow is a
distinct hissing or buzzing sound."



St. Elmo's fire and corona discharge are distinctly different, although
St. Elmo's is often described as corona, it actually isn't.

Yes, corona in high fields does sound like hissing or buzzing. Anyone
who has been on a high mountain in a thunderstorm has probably
experienced this, and even more exciting is when you get corona
discharge off your hair standing on end.

St Elmo's is a bit different. It has a distinctly different appearance:
a sort of bluish glow that extends some distance surrounding the object,
as opposed to corona, which seems to be "stuck" to the surface. And then
a high field streamer discharge is still different. If you've seen both,
it's noticeably different, but hard to describe.

As best can be determined, St Elmo's arises when the surface of an
object is wet and in a field. The water sprays off the surface (much
like is done in electrospraying), and the droplets carry charge away.
Several different things then happen: 1) The charge causes the droplet
to break apart into smaller droplets from electrostatic forces; 2) the
field at the surface of the droplet is now too high and the air
breaksdown at the surface of the droplet, discharging it, and 3) the
droplet starts to shrink from evaporation, getting smaller, so the field
at the surface grows, etc.

So St. Elmos is not really corona off the object, but corona/breakdown
on the droplets being electrosprayed off the surface.

It is possible to generate St. Elmo's in the lab on the surface of an
object with fairly large radius of curvature (i.e. that would NOT be
subject to surface corona) in a moderate field. If someone wants to try
an experiment, put a wet cork or dowel of wood in a shallow tray or
puddle of water with a large flat electrode suspended over it to create
the overall field. A field of 10kV/cm will work quite nicely. I used
two baking pans separated by cut down styrofoam cups driven by a small
modular HV supply (50kV@ a few mA) driven by a variac. Make sure you
have a resistor in series with your HV supply, because occasionally
you'll get a flashover, and you want to limit the current.