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...
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.
USEFUL LINKS
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 antics...build his microwave "herf gun" only if you know
what you are doing!!
