Effect of Lightning on Thin Metal Surfaces

In the search for means for measuring the properties of nataural lightning much can be learned from the evidence left at points struck by lightning. This paper deals principally with such evidence and the process of evaluating the characteristics of the lightning strokes responsible for the evidence. For six years a nickel-plated, 18-inch copper sphere, 878 feet above the ground atop the WSM radiator at Nashville, had been collecting data in the form of holes and pits due to lightning strokes to the sphere. A total of 150 holes of varying sizes were found together with 300 pits. A laboratory setup was made consisting of the high-capacity impulse generator together with a d-c generator so arranged that the known characteristics of lightning with respect to the so-called continuing current could be duplicated. With this equipment holes having the same appearance could be produced in copper sheets, and from the results of the test a calibration curve was produced. At the same time calibration curves were obtained for other metals in addition to copper. From these data an expectancy curve was obtained between coulombs and per cent of holes. The average hole corresponded to 15 coulombs while the maximum size hole corresponded to 240 coulombs. No satisfactory calibration was obtained for the pits, many of which no doubt were the result of high current peaks having a low coulomb content.