Point‐discharge currents from natural and artificial points

When point discharge occurs at an earth‐connected point, ions of one sign travel to earth through the point while ions of opposite sign travel away in the wind. This stream of ions reduces the potential gradient at the ground to leeward of the point below that to windward. Theory shows that, under suitable conditions, this difference in potential gradient is close to that predicted for a line of charge, even though the line is spread into a cone by turbulent diffusion. Measurements with point discharge from a 34 m mast show good agreement with theory, and this makes it possible to use the difference of potential gradient to determine the point‐discharge current through a tree. When applied to a line of trees, seasonal differences are found suggesting smaller currents from trees in leaf than from bare trees. Measurements with a 12 m sycamore tree in full leaf show no point‐discharge current even in potential gradients up to 7,000 v/m.