Measurements of atmospheric point discharge currents

Abstract

Investigations of the behaviour of point discharge currents down an earthed point mounted at the height of surrounding trees are described. Simultaneous measurements of the potential gradient at the ground to windward of the point, point discharge current, and windspeed at the point indicate that the current can be represented by the expression I = k (W + C) (F-M) where I is the point discharge current W is the windspeed F is the potential gradient M is the onset value of the potential gradient and k and 0 are constants. This is in direct agreement with the results of Chalmers and Kirkman (1957) for a point on a high mast and considering what has been said by Chalmers (1957) the results suggest that the current flowing down points in all conditions can be represented by the equation I = A (V -Vo) (W(^2) + C(^2)V(^2)1/2 where I and W are as above. V is the potential of the air surrounding the point Vo is the onset potential and A and c are constants. Measurements of point discharge currents down a tree indicate that for a fixed windspeed the current can be represented by I = a (F - M)and that other things being equal the current down the tree is only one tenth of the current down the point. This suggests that estimations of charge brought down by point discharge based on the equivalence of a point and tree would have to be greatly modified. Laboratory experiments were carried out to test the practicability of measuring currents down trees by a transformer method and currents of 5 microamps were measured with ease. The sensitivity could be greatly improved and it is suggested that this is a possible method of measuring point discharge currents down trees.