"Jim Lux" wrote
...
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 , 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.

May be that more water bigger "distance surrounding the object".

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.

All this is fascinating, especially with presence of water.

All this phenomenon occur at higher voltages. In normal transmissing no such
spectacular glows but only simple field emission.

In textbooks is wrote that the electron emission is more effective than the
absorption and that the glow is different.

Do you know how is the netto flow (field emission/absorbtion) of electrons
during transmitting?
S*

..