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DISCUSSION 
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Discussion 
Mr. C. E. Anderson—I am sure most of us 
had not been aware of the importance of the 
rainbow as a possible means of getting informa- 
tion on raindrop sizes and on oscillations of the 
drops. 
Dr. Bernard Vonnegut—I think it is possible 
that raindrops and rainbows may be affected 
not only by shock waves but also by the sudden 
changes in electric field caused by lightning. The 
raindrop shape should be altered by an electric 
field, and it would be most helpful if we could 
use the rainbow as a tool for measuring the elec- 
tric field. The usual measurements of the electric 
field in thunderstorms are complicated by the 
fact that most instruments profoundly modify 
the field being measured. If we could use the rain 
itself as an indicator of the electric field it would 
be an ideal measuring device. 
Dr. F. Volz—But this vibration of the rain- 
bow is not synchronized with the lightning, but 
rather with the thunder. 
Dr. C. L. Hosler—My next-door neighbor is 
from Berlin and several times when there was 
fog he observed a variation in brightness that 
seemed to coincide with the 16-cycle alternating 
current of the electric overhead powerline. There 
was no visible discharge anywhere from the 
powerline to the ground. Can you explain this 
to me, so I might explain it to my neighbor? 
Dr. Heinz Kasemir—This phenomenon is well 
known and can be observed at any railroad sta- 
tion where steam engines are operated and elec- 
tric locomotives are fed from an overhead line. 
As soon as the steam from the steam locomotive 
comes into the neighborhood of the powerline, 
which usually runs on a very low frequency 
(1624 eps at Munich, Germany) the steam cloud 
shows an oscillation in brightness in synchronisa- 
tion with the frequency of the powerline. This 
problem was studied by G. Escherich of the 
Technische Hochschule, Munich, and the results 
are published (see review of this paper by A. 
Schmauss in Meteorologische Zeitschrift, 57, 
83-85, 1940). The steam droplets, contaminated 
by soot particles, form an electrical dipole and 
oscillate with the alternating electric field. The 
optical impression is that of a periodic variation 
in brightness. 
