Point discharge in atmospheric electricity

Abstract

Investigations of the potential gradient beneath the ion stream liberated in point discharge in the atmosphere are described. The electrostatic effects of the ions are analysed theoretically and the results of simultaneous measurements of the potential gradient on the upwind and downwind side of an artificial point, wind speed, wind direction and temperature gradient reported. It is concluded that for distances from the base of the point less than √3 h times the height of the point and excluding conditions of snow and very heavy rain the electro-static field of the ion stream can be computed on the assumption that the ions take up an infinite line distribution. These conclusions are then applied to measurements made on a line of trees at Durham University Observatory and around a sycamore tree at the Durham City Golf course. The results for the line of trees indicate a change in point discharge characteristic with season; for the sycamore tree the current down the tree was found to be less than 1μa for a potential gradient of + 7,000 V/m. The changes of characteristic with season might be correlated with the appearance of the leaves on the trees. The implications of these results are discussed with reference to the assessment of point discharge current density basedon the equivalence of an artificial point and tree of a similar height. It is concluded that this equivalence is not in general justified and that a more comprehensive study of a similar nature is required before a reliable estimate of mean point discharge current density can be obtained from measurements with artificial points.