Single Spark Photography and its Application to some Problems in Ballistics

Abstract

SPARK photography, in which the illumination is provided by an electric spark of such short duration that a moving object appears stationary, has many applications in the investigation of high-speed phenomena. The record obtained is not an image, no lens being used, but is simply the silhouette of objects between the light source and the photographic plate. Two distinct problems are presented in the photography of moving objects. One of these is the timing of the spark so that the desired epoch of the phenomenon under investigation may be photographed, and the other has to do with the duration of the spark. All the photographs described in this article were taken on plates not larger than 8 by 10 inches. A projectile moving at a speed of 2700 feet per second would be in front of such a plate, and therefore in a position to be photographed, for only 0.0003 second. If the projectile is to be photographed within an inch of a predetermined position, the time of occurrence of the spark must be correlated with the position of the projectile to within 0.00006 second.