Point discharge pulse measurements in atmospheric electricity

Abstract

Previous methods for estimating the charge transferred to earth by point discharge are reviewed. It is concluded that point discharge on trees may well make a large contribution to this transfer of charge but that no reliable method of measurement has previously been duvised. A laboratory study of the pulsed nature of the point discharge currents produced at metal points and natural points has been made. It is concluded from these measurements that the amplitude and repetition frequency of these current pulses can be used to give the total discharge current. A new method for measuring point discharge currents in trees has now been developed and the apparatus successfully calibrated. This takes the form of a capacitative electrode, attached to a branch at the top of a tree, which detects pulses of point discharge current in the tree. The electrode is connected to an electronic pulse detector which gives a measure of the point discharge current in the tree. Measurements of point discharge currents have been made in two trees over a period of three months in a plantation of conifers in Weardale. The potential gradient and the point discharge current through a metal point were measured outside the plantation over a period of eleven months. The charge transferred to earth by point discharge on one tree in three months was found to be 4.3 x 10(^-4)C whereas for the metal point in the same period the charge transferred was 72 x 10(^-4)C. It is concluded that this difference is a result of the higher potential gradients necessary to initiate point discharge on the tree compared with those for the metal point. The point discharge measurements for two trees, one on the edge and one inside the plantation, are compared and found to be of similar magnitudes. The charge per unit area transferred to earth by point discharge on all the trees in the plantation is estimated to be 270 C km(^-2) yr(^-1).