The Measurement of Lightning Currents in Direct Strokes

The magnitude and wave shape of the discharge currents in 46 direct strokes have been measured during a five-year period at 25 direct-stroke stations on structures ranging in height from 75 to 585 feet. Measurements have been made with instruments capable of recording the wave front and tail of the high-current peaks and the low-magnitude continuing currents to as low as 0.1 ampere. These data are correlated with 138 special camera photographs of strokes to open ground obtained in this investigation, with similar photographs taken in South Africa and with stroke current measurements made on the Empire State Building in New York City. Most strokes to structures, less than 600 feet high, appear to have discharge characteristics essentially the same as those to open ground. Their probability of being struck is about proportional to height, being in the order of two strokes per year for 500-foot structures and 0.3 per year for 100-foot structures. Whereas practically all strokes to the 1,250-foot Empire State Building are initiated by an upward streamer propagating from the building to the cloud, only four such strokes have been recorded in this investigation: three out of 13 strokes to a 585-foot stucture and one out of 13 to a 360-foot structure. Strokes of this type, as recorded here and to the Empire State Building, had discharge currents consisting principally of a continuous flow of low-magnitude current upon which the high-magnitude short-duration peaks are superimposed.