5*4 



NA TURE 



[October 14, 1922 



TNSTANTANEOUS photography by means of an 

 J- electric spark provides the investigator of high- 

 speed phenomena with a must valuable source of data. 

 Smii photographs are of the shadow variety, the bullet 

 shadow being projected upon a photographic plate by 

 a spark of great intensity and short duration. If the 

 bullet is moving with a speed equal to or greater than 

 that of sound, it propagates from both its nose and base 



Photography of Bullets in Flight. 

 By Philip P. Quayi.e, Assistant Physicist, National Bureau of Standards. U.S.A. 



possible, because the potential available for the photo- 

 graphing spark is not limited by the dielectric strength 

 of some trigger gap of fixed and small dimensions. 



The regulation of the potential of the spark is essential, 

 however, since great irregularities in time occur when 

 the apparatus is not operated at the same potential, 

 the spark occurring earlier or later than the transit of 

 the bullet across the plate. This, of course, precludes 

 satisfactory records when working witli modern high- 

 speed bullets. When the proper potential has been 

 attained a signal light is automatically turned on. 



No lens system is employed in the apparatus. An 

 arrangement which has been found very satisfactory 

 places all of the photographic apparatus, except the 

 trigger, inside a small light-tight house. 



The trigger itself is located outside the house and 

 near the trajectory. This trigger is an interrupter of 

 the type used by the French in connexion with the 

 Joly chronograph. The use of this instrument and the 

 type of springs used in the photographing-spark-switch 

 were suggested by Dr. D. C. Miller, of the Case School 

 of Applied Science, where the apparatus was developed. 

 The trigger consists of a metal diaphragm about 2 

 inches in diameter enclosed in a circular metal box. 

 The diaphragm forms one side of an air-tight enclosure, 

 and on the inside face of the diaphragm is attached 

 a circuit-breaking mechanism. This circuit-breaker 

 functions when the crack wave emanating from the 

 bullet strikes the diaphragm, which in turn throw- bark 

 a small hammer, thus interrupting the circuit and 



1 ' 'pervitins triu^er. 



a compressional wave. Light from the photographing 

 spark in passing through the denser atmosphere of the 

 compressional wave is refracted as by a lens, so that the 

 wave front is also projected upon the photographic 

 plate with the bullet. The method lends itself readily 

 to the investigation of a projectile's stability at various 

 points along its trajectory and to many allied problems 

 of exterior ballistics. Instructive photographs of the 

 recoil and shell ejection of automatic rifles, pistols, and 

 machine guns may also lie obtained in this manner. 



Among the must important of the early contributions 

 in this field of research are the admirable spark photo- 

 graphs by Prof. ('. V. Boys (Nature, Vol. 47, pp. 415 

 and 440). who greatly simplified the elaborate apparatus 

 of Prof. E. Mach. In Prof. Boys's apparatus the bullet 

 was employed to close the spark circuit, and this 

 method has been followed in experiments which have 

 been carried on since that time, so far as the present 

 writer is aware. 



In the method described in this article the setting-off 

 or triggering ol the electric spark by which the photo- 

 graphs are taken is controlled by the compressional 

 wave produced by the flight of the bullet, so that no 

 wires or other portions of the apparatus need appear on 

 the plates. Since the sound wave is used to trigger 

 the photographing spark, the position of the rifle firing 

 the bullet may be varied at will without affecting the 

 functioning of the apparatus, the only requirement 

 being that the bullet shall have a speed greater than 

 ili.'t "I nu ml. When the speed of the bullet is less than 

 that of sound the muzzle blast may be used to trigger 

 the spark. In such cases the rifle must not be moved. 



In Prof. Boys's type of apparatus the photographing 

 spark is set off by the closing of a secondary gap by the 

 bullet itself. In the present apparatus a much more 

 powerful spark may be used than would otherwise be 



NO. 2763, VOL. I io] 



tripping the photographing-spark-switch, with which it 

 is connected in series. 



The essential parts of the photographic apparatus 

 consist of a large Leyden jar battery charged by a static 

 machine which is motor driven, the control switch for 

 the motor being mounted on the table with the rifle. 

 A potential regulator which is connected across the 

 battery functions when the proper potential has been 

 reached, and trips a switch which disconnects the 

 battery from the charging machine and short-circuits 

 the terminals of the latter. The switch when tripped 

 turns on a signal light located outside the apparatus 

 house as a signal to the rifle operator to fire. 



When operating the apparatus the general sequence 

 of events is as follows : 



