Re: Ball lightning - Terry's thoughts....

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: William Beaty <billb@xxxxxxxxxx>

On Thu, 11 Aug 2005, Tesla list wrote:

> Original poster: Scott Stephens <scottxs@xxxxxxxxxxx>
>
> Two things to consider before you discount what your physics student:

But remember: theory must be tested against nature, or in other words,
experiment trumps theory.  Since 900 watt microwave ovens DO easily create
corona, any arguments which say differently are in error, and are only
made in ignorance.  If those physics students weren't clueless about the
many interesting corona effects common in microwave ovens, they wouldn't
dream of suggesting that corona is impossible, instead they'd hunt for
their error.  The error is the one I mentioned: since corona only appears
when the oven is run with an extremely light load, i.e. when Q is high,
this suggests that their math could be correct, but they wrongly imagined
that the microwave beam was propagating in just one direction, and failed
to take into account what happens with standing waves and resonance.


Once I made a corona candle: I sharpened a 1/4" battery carbon in a pencil
sharpener, cut it down to 1" long, stuck in a drilled hole in a hunk of
brass, and placed in on the glass rotor dish.  The tip of the carbon made
a white flame about 1/2" tall, but it took some repositioning of the
carbon to get it to ignite.   Also, when I ran the oven a second time, no
flame appeared.  I discovered that the sharp tip of the carbon rod had
become rounded down: it was being consumed by the high-power discharge.

Even simpler: make a square of aluminum foil a few inches across with torn
edges.  Crumple it slightly, then tape one corner to the glass rotor dish
in your oven (so the fan doesn't blow it around.)  Nuke it in a
high-wattage oven and wait for blue-white corona with a loud 120Hz buzz.


> 1. What is the impedance of a microwave oven cavity?

For locations outside the nearfield region of the cavity walls (farther
than 1/4 wavelength away, or about 1",) the impedance will be the same as
free space.

> 2. What is the Q of that cavity?

No idea.  It's going to be proportional to the percent of the wave
absorbed upon each reflection from the walls, which would depend on
resistivity of the steel.  Thickness of the steel probably doesn't matter,
since skin effect will dominate, I think.  Steel is a relatively poor
conductor compared to the standard for UHF/microwave:  silver plated
copper.  Hmmmm!  I have a roll of 1mil copper foil.  I should try adding
this to the interior of the oven to improve the Q.  Or perhaps make a
partial box from copperclad PCB.


> MO frequencies are 2 to 3 orders of magnitude higher than TC
> frequencies. For comparable Q's, (IIRC single-mode waveguide filters
> have Q's in the 100's) I'll SWAG the resonant voltage increase might
> be 10 to 30 times the nominal value, up to perhaps 1000 volts. Not
> enough for corona in air.

Corona in air doesn't depend on absolute voltage, it depends on field
strength, i.e. depends upon electrode sharpness *and* absolute voltage.
Also, the work-function of metals figures in as well, so you need to
subtract about 500V when calculating the e-fields produced by a particular
electrode shape at a particular voltage.

It might be interesting to place an adjustable sphere-gap in the oven and
see how far a spark would jump.  Without having tried this, I'd give a
seat-of-pants guess of 1mm or so.



> Since MO plasmas usually require a conductive carbon or metal,

The plasmas leap from the sharp edges of the metal objects.  For example,
if you fold over the sharp edges of aluminum foil, you don't get any
corona outbreaks.  But if you intentionally create sharp edges and points,
the corona always appears.


> ionization from the short UV from an external arc source, I suspect
> your physics student is right about kWatt-class MO's inability to
> ignite a plasma in air, even if argon.

Lol!  WHen immersed in argon in a microwave oven, the edges of a piece of
torn aluminum foil spews out blue-white plasma filaments many inches long!
This is one of the more fun microwave oven demos that I perform.

> Perhaps somebody should do some math?

Personally, I'd much rather be Michael Faraday than be Charles Proteus
Steinmetz.  Even though he was clueless about the mathematics behind
electricity, I think Faraday had more fun.


(((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))
William J. Beaty                            SCIENCE HOBBYIST website
billb at amasci com                         http://amasci.com
EE/programmer/sci-exhibits   amateur science, hobby projects, sci fair
Seattle, WA  206-789-0775    unusual phenomena, tesla coils, weird sci