Currents carried by point-discharges beneath thunderclouds and showers

Abstract

1.— Introduction. The present paper describes experimental methods employed for investigating the sign and magnitude of the vertical current due to the point-discharge of electricity from an elevated metal point, during the intense electric fields associated with thunderstorms and showers. The results of preliminary observations extending over a period of about eight months are tabulated. It is found that there is a considerable net transfer of positive electricity upwards from the earth to the atmosphere by the discharge currents. The vertical discharge current is thus in the opposite direction to the normal fine-weather ionization current and to the convection current carried by various forms of precipitation. A comparison of the magnitude of the observed quantities with that of the fine-weather current indicates the important part played by the point-discharge currents in the total exchange of electricity between the earth and the atmosphere. Finally, the distribution of electric field below a cumulo-nimbus cloud deduced from the observed variations in magnitude and direction of the current, as the cloud passes overhead, strongly suggests that the great majority of shower-clouds and thunder-clouds observed were bipolar clouds of positive polarity, the upper charge was positive. 2.— The Discharging System. The discharging apparatus is carried at the top of a vertical wooden pole, whose base is attached by a hinge to a stout wooden stake, fixed vertically in the ground. The pole can thus be lowered to the ground for inspection of the insulation. When the pole is in its vertical position, the height of the discharging point above the surface of the ground is 8·3 metres. The lead carrying the current to the measuring apparatus consists of about 30 metres of lead-covered rubber insulated cable, of external diameter 5·6 mm. The able travels down the pole and thence, underground, to the hut containing the observing apparatus. The distance from the base of the pole to the hut is about 14 metres. Any disturbance of the field at the top of the pole, due to the presence of the hut is negligible.