Page:Popular Science Monthly Volume 91.djvu/380

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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

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