364 



Popular Science Monthly 



A novel effect in photographing lightning 

 by holding the camera in the hand 

 and giving it a side -to -side motion 



the moving camera, artificial electrical 

 sparks in the laboratory, and showed 

 that such sparks begin with a brush 

 discharge from each 

 of the electrodes, 

 these partial dis- 

 charges gradually- 

 ionizing the air (i. e., 

 making it a conduc- 

 tor of electricity by 

 breaking up its mole- 

 cules into positive 

 and negative "ions" 

 or electrified parti- 

 cles), between the 

 electrodes, until finally the whole inter- 

 val is bridged over by the spark. Some- 

 thing similar appears to occur in a lightning 

 flash. 



It is obvious that if a discharge of 

 lightning has a sensible duration, the 

 rotary movement of the camera will 

 spread out the flash, as impressed on 

 the plate, into a more or less broad 

 ribbon. Most photographs of rib- 

 bonlike streaks of lightning are, in 

 fact, due to the accidental movement 

 of the camera during exposure. 

 When the camera is held in the 

 hands, the occurrence of a flash com- 

 monly causes the photographer to 

 give an involuntary start, and this 

 explains not only cases of ribbon 

 lightning, but also various other 

 peculiarities of ordinary lightning 

 photographs. A certain amount of 

 spreading in the flash is, however, 

 due in some cases to "halation." 



The splendid photographs of Walter 

 with a camera rotating at a known rate 

 enable us to make an accurate time 

 analysis of the details of the flash. 



How Long Does a Flash of 

 Lightning Last? 



Such photographs show that some flashes 

 are practically instantaneous, while others 

 may last as long as half a second or more. 

 When flashes of the latter class are photo- 

 graphed by Walter's method, the resulting 

 picture shows several parallel streams of 

 light, proving that a number of successive 

 discharges occurred along the same path. 

 These give to lightning its flickering 

 appearance. 



A photograph of this type taken by 

 Larsen shows forty distinct discharges in a 

 single flash, at average intervals of 0.0156 

 seconds, the total duration of the flash 

 being 0.624 seconds. Photographs of this 

 character also frequently show the pre- 

 liminary partial discharges. 



Walter improved 

 the moving camera 

 by the addition of a 

 stationary camera 

 with which exposures 

 are made at the same 

 time, in order to 

 show the actual di- 

 rection of the flash 

 from the observer. 

 The same investiga- 

 tor has more recently 

 developed a stereoscopic process of photo- 

 graphing lightning. Two stationary came- 

 ras are set up side by side, several feet 

 apart, and pointed in the same direction. 

 When a flash is photographed, its position 

 on the two plates is different with respect 



Growth of electric spark discharge, illus- 

 trating the way in which the lightning 

 flash builds up its path through the air 



Photographed simultaneously by two cameras about 

 six and one half feet apart. By this method the 

 actual distance of the lightning can be determined 



