BULLETS IN FLIGHT

Evening Post, Volume CXIII, Issue 30, 5 February 1927, Page 20

BULLETS IN FLIGHT

HOW, THiHYARE PHOTOGRAPHED

! The first spark phootgraphs of projectiles in. flight were taken by Professor Ernst Mach, of Prague (who died in 1916)- about 1881. Mach arranged his apparatus in such a way that an electric spark appeared in one gap when a second gap was closed by the bullet passing through it. A lens behind the first-mentioned gap threw "the light on the lenß of a photographic camera focussed on the second gap, a picture of the bullet and the wires being obtained in this way. Mack's idea was that the compression of the air in front of the bullet, and the rarefaction behind it, should reveal themselves when illuminated by the spark by the changes in the refractive power of the air; his photographs showed the expected features. In order to render these changes more striking (says "Engineering"), he arranged his camera for the striae photography method of A. Toepler, the inventor of the induction electric machine used in theso experiments. Eesuming the ballistic investigation in 1893, Professor C. V. Boya dispensed with the photographic camera, and took, by the light of a strong spark, a direct shadow picture of the bullet at the moment when it was in front of the photographic plate. His triggering device was similar to that of Mach, and his photographs also showed the bullet and the wires. In his recent experiments Mr. Philip P. Quayle, of the United States Bureau of Standards, also took shadow photographs, but, in his experiments, the bullet does not close a contact, and is not interfered with in any way, and he has been able to follow a rifle bullet in as many as seven stages, from the muzzle to a distance of llin in front of it (and farther away, of course), all within 0.0005 second. The spark has to be timed to appear just at the moment when the bullet is in front of the plate, and the spark must bo instantaneous to give a sharp shadow picture. The plates are Bin by lOin. A projectile moving at 2700 ft per second would be in front of tho plate for 0.0003 second. Some camera shutters are Baid to operate within 0.0006 second; in reality they are raroly faster than 0.002 second. That rate would be sufficient to photograph a racing car at 120 m.p.h., which, at this shutter speed of 0.002 second, would travel 4in while in front of the plate. But if the projectile is to appear on the plate within an inch of its predetermined position, the correlation between thfl spark and the position must be within 0.00006 second, and the exposure should not exceed 0.000002 sec°.?fl £? Produce a ?hfiEP image.