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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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