THE CONCEPTIONS of lightning and of lightning discharge currents through arresters have been altered and enlarged by recent studies. Until not long ago the most complete information available on the magnitude and wave shape of lightning surges on electric systems has been that obtained by the cathode-ray oscillograph investigations of lightning voltages, reported and discussed before the AIEE,1 and elsewhere. Besides this there exists a large mass of data on the crest magnitudes but not the durations of currents in lightning strokes and arrester discharges. Based on the data available two conclusions have been generally held: one, that in a small percentage of discharges, the currents may be high, perhaps 65,000 amperes or more; and two, that the total duration of the discharge might be of the order of 100 microseconds, but not much more. Lightning-arrester design was based principally upon these conclusions. Arresters available during the past six or eight years have in general been able to discharge high currents of the durations cited without damage. In the field, the over-all experience with such arresters has been good. Throughout the country the average failure rate of Autovalve distribution arresters manufactured during that period has been less than a quarter of one per cent per year. However, in rural locations in some parts of the country failure rates of three to four per cent have occurred. This is considered excessive. The variation in failure rates led to the suspicion, several years ago, that there are characteristics in arrester discharge currents that had not been taken into account and which are themselves influenced by such factors as lightning exposure, stroke characteristics, ground resistance, and system characteristics. It is true that to some extent the significance of the data may be obscured by failures from abnormal system voltages and by the performance of obsolete arresters, but these factors were not sufficiently weighty to explain the observations.