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Tornadoes as electrical machines
Excerpt from Homemade Lightning
by R.A. Ford
Several years ago, I experimented
with the effects of high-voltage direct-current discharges onto
moist semiconducting surfaces. The substances including granite,
marble, agate, limestone, sandstone, white chalk, plaster of paris,
slate, and unglazed clay were chosen for their fine porosity and
ability to absorb moisture.
A most unusual discharge presented itself while I was working with
unglazed clay flowerpots. Figures 19-2 and 19-3 illustrate the setup
for producing miniature electric tornadoes with a white fireball
tip. A 1-inch-long section of plain steel piano wire, 0.015 inch in
diameter, is positioned 'k inch from the clay surface. The pot sits
in a shallow metal pan with water covering the bottom. The power
supply is a full-wave transformer rectifier that has an output of
10,000 Vdc at about 0.7 to 1.0 milliamp (7 to 10 watts). The wire is
made positive, and the pan is negative.
The unglazed clay's properties are crucial. At the Ceramics Research
Laboratory, University of Illinois, analysis comparing clay pots
indicates that the acceptable clay, on which the vortex forms, has a
dry surface resistance of infinity and, when dipped in water and the
excess wiped off, a surface resistance of 300,000 ohms. Place the
probes of the ohmmeter 1 cm apart for this indication. The clay
color is light red; that means it contains a smaller percentage of
iron oxide. (Thanks to Dr. Relva Buchanan for determining the
characteristics of the clay samples!)
Moisten the selected flowerpot with your finger dipped in water and
place it as shown. Turn on the power and the discharges will remove
moisture, increasing the clay's resistance. In the dark, when the
fireball forms, the discharge either squeaks or is silent. With the
positive wire near the clay, a dazzling pure white ball, about 1 mm
in diameter, will form on the pot in an electric tornado-vortex,
varying between 'A and 3/8 inch long. The ball will slowly traverse
the surface in a sinuous movement, seeking out a path of preferred
resistance.
When I examined the ball through a #4 gas welder's filter, rays from
the fireball were still visible. The amazing thing to me is that the
heat from the tiny fireball was so great that it permanently etched
a black path into the clay. Figures 19-2 and 19-4 show the
characteristic track signatures. When the polarity is reversed, the
tip of the steel wire () often glows white hot, and the clay remains
cool.
How does this unusual discharge relate to real tornadoes and
waterspouts? Many good descriptions of tornado lights with
fireballsand internal lightning bolts exist, but I found only one
case with a fireball maintained at the terminal end (as in our
experimental condition). One rare account in Fig. 19-6 is from the
British
journal, Weather (1949).
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