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Re: [TCML] St Elmos fire isn't corona


Interesting, I'd always thought St. Elmo's Fire was purely a HV "wind" discharge from items in a large electric field. e.g. corona. Never really thought that there was other effects happening too.

    I will try the experiment and see if I'm convinced :)


On 24-Feb-17 15:23, jimlux wrote:

St. Elmo's Fire isn't really a corona phenomena, at least how we all think of corona as discharges from a body at a high voltage.

If you read descriptions of St. Elmo's Fire, it's more like a glowing sheath around wooden masts and spars during a thunderstorm.

A colleague of mine, Stephen Fuerstenau, explains that St. Elmo's is actually a much more interesting phenomenon.. It's a combination of electrospray and corona breakdown on the droplets being sprayed.

So here's what's likely going on - the surface of the object is covered with water (often salt water, but water that has some conductivity in any case). The water is charged, along with the object it's on. Small irregularities in the surface of the water causes droplets to be "spit" away (think of the demo where you put a bucket of foam peanuts on the topload of a Van deGraaff generator). As a bump develops in the surface of the water, the surface charge is repelled by the surface charge on the surrounding water, which causes the bump to get bigger, stretch, and eventually form a separate droplet, which is then repelled, just like the foam peanuts on the VDG.

As it happens, the surface charge in this situation creates a field that is right at the limit for breakdown (the charging of the big object ensures this). As the droplet moves away, the surface field actually increases because of two reasons: 1) the potential difference between the droplet and the surroundings increases... if the object from which the droplet left was at 20kV, and the droplet starts out at 20kV, the potential difference is small, and as the droplet moves away, the potential difference increases; 2) the droplet evaporates, making it smaller in diameter, but the charge remains the same, so the field goes up. Both cause a surface breakdown (corona, if you will), and the droplet is now glowing.

This phenomenon can occur over a wide area, and on things that aren't particularly "pointy".. the small radius of curvature is in the droplets, not the big charged object.

An experiment YOU can do if you have a suitable HV DC supply (it might work with a NST's AC output, but I've not tried it).

Set up a metal pan with a thin layer of water in it - that's your ground plane. in the middle of the pan, put a cork or rough block of wood that just sticks up a bit above the level of the water, making sure the cork/wood is "wet". A nice dense sponge might also work.

Have some insulating supports, and put another flat electrode (another cookie sheet or pan?) above the bottom and put a voltage between the two. If the field is high enough, you'll see the water come up the wood and form tiny droplets (a bright light from the side makes it easy to see them). get the electrospray working first.

Then, turn out the lights and behold your St. Elmo's fire generator.

There are some fiddly bits here - if there's much air movement it blows the droplets away - you'll likely get some flashover arcs when you're setting up - it *is* HV - resist the temptation to reach in and move things when the power is on.

I used a 50kV DC supply with a big resistor in series to limit the current.

I think it might work real well with a Van de Graaff generator with a suitable setup.

I've not fooled with changing the salt content of the water or a variety of other factors - this would be a "most excellent" science fair project, by the way.

The wikipedia article is kind of ok, but has some incorrect data (air does not break down in a field of 100kV/m, more like 3MV/m. near small radius of curvature objects, the field *is* >3MV/m, which is why it breaks down)

However, the description from Dr. Braid about the glowing horse ears and hat brim is a good one.
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