Differential mill for measuring electric fields and conductivities in thunderclouds

Summary A differential rotating mill for the measurement of electric fields has been designed and built. The instrument was specifically designed to be insensitive to the effects of space charge and frictional charging so that it could operate in the environment of an active thundercloud. The electric field range of the device is 100 to 250,000 volts per meter, (however, corona should occur before the maximum measurable field is reached) and the conductivity range is approximately (depending on the electric field) 10^–11 to 10^–13 mhos/meter.Forty of these devices have been parachuted into active, and potentially active, thunderclouds around Tucson, Arizona. Although the presently available measurements are too few for an unequivocal conclusion, the data strongly support the presence of very high conductivities in thunderstorms.This research was supported by the Atmospheric Sciences Section, National Science Foundation under Grant GA-701.