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Ball Lightning
in microwave ovens

Ever since the invention of the microwave oven in the 1950's,
humans have had a fascination with  "nuking stuff ".  As a
result, we now know of all kinds of interesting effects that can be brought
about by placing non-food items such as CDs, burning toothpicks,
metal items, lightbulbs, cats, etc. in our microwave.  Sometimes,
bizarre things happen and a flaming ball forms in the oven,
surviving several seconds and rising to the top of the oven where
it then dissipates.  Folks have found their own means of trying to
reproduce the "ball lightning"- and now I share my own ideas with you!

This is the "ball lightning" effect in the 700-watt oven in my dorm, captured
via a video camera.   The ball is on the order of an inch in diameter and
glows a fiery orange / violet color.  It survived about four seconds, and was
produced via Procedure 1 below...

First a bit of theory...a microwave oven is a conducting cavity that confines the 2.54 GHz energy produced
by the oven's magnetron.  The magnetron is a really clever type of vacuum tube that converts DC into
microwaves with over 70% efficiency.  It usually resides in the right-hand side of your oven , coupled to
a short length of waveguide that leads to the oven cavity.  A typical oven magnetron develops 700 watts of power.
The waveguide that leads from the "maggie" to the oven can only propagate a single mode of oscillations- the
simplest transverse electric mode.  But the oven itself is much larger than the dimensions of the wavelength of
the microwaves, and many field configurations are possible.  The energy is distributed among these modes.
The fields in a microwave oven are quite high- voltage gradients of 500 V / cm exist in an average oven,
according to a book on microwave heating technology (sorry- I forgot the name!)  So this makes it possible
to generate arcs and discharges with the RF power in the oven.

The reason the "ball lightning" is formed is that hot ionized gas in flames and arcs is a pretty good conductor,
and like a moist hunk of meatloaf, will absorb microwave power and resistive heating will result.  Being hot and
lighter than surrounding gas, the arc or flame will rise and can detatch from its source.  The resulting ball is
kept hot by continuing to absorb microwaves.  When the ball reaches the metal walls of the oven cavity, its heat
is absorbed and it loses its ability to conduct.  Hence the discharge is terminated.

People can do two things to initiate the ball:  they can provide a flame to absorb microwaves, or they can provide an electric arc.  The flame itself is a little difficult, since it needs to be put in the right place in the oven's fields to absorb sufficient power.  So usually the toothpick, matchhead, flaming bread, candle, etc. approach will take some playing around before you can get a ball on a regular basis.  But my devices are designed to put the high fields right where the flame is.  They are quarter-wave resonators that develop a voltage antinode at the free end.  My first design generates an arc from this point, which then develops into a ball.  My second design uses a flame at the end of a quarter-wave element.  Note that, as I said earlier, the wavelength in the oven is different from that in free space, and so my dimensions may be off.  But the devices work well...

This is Design 1.  Cut a disk "B" of aluminum or copper, 2.4 inches in diameter.  Solder or tape a 1.5-inch long piece of metal sheet "A" to its midpoint, cut a sharp point at the free end, and bend it within a few centimeters of the disk edge as shown.  Aluminum foil works, but it's not durable, as you can easily observe after firing the toy.  If you value the nice paint job in your oven cavity, don't set the toy directly on the bottom of the oven, but rather on a glass or other standoff so it can't spark through your paint and blacken it.  The toy works best if it's not rotated (disable your oven turntable) and if your oven doesn't have a "mode stirrer" fan.  If it does, you can disable that also with a rubber band (find out how yourself.)  This device wants a powerful oven, 700 watts or more.

Design 2.  This is made from copper pipe and sheet that is silver-soldered together to make a little alcohol lamp.  The dimensions of the surface "B" are the same as the dimensions of "B" on the first device, and "A" is a piece of 3/8" refrigerator tubing about an inch long.  A little ethanol is poured into the lamp through the tube, and a piece of string is inserted to wick up the alcohol.  Light the lamp (please make sure it is not leaking any alcohol!!!) and place in oven.  The flames grow and become balls.


AMI PARTSis a distributor of microwave oven parts such as magnetrons.  Good prices on diodes at $6 ea...
SAM BARROS has a page that describes some fun microwave his microwave "herf gun" only if you know what you are doing!!

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