BENJAMIN Franklin, more than 200 years ago, proved that lightning was an electrical discharge and measured the sign of the cloud charge that produced it [l]. Modem research on the physics of lightning began in the early 20th century with the work of C.T.R. Wilson [2], [3], the same scientist who received the Nobel Prize for his invention of the cloud chamber. Wilson, by making and analyzing remote measurements of thunderstorm electric fields, was the first to infer the charge structure of the thundercloud and the amount of charge involved in lightning