SPARK PHOTOGRAPHY, in which the illumination is provided by an electric spark of such short duration that even the most rapidly moving objects appear stationary, has many applica tions in the investigation of high speed phenomena. The record ob tained is not an image, no lens being used, but is simply the sil houette of objects between the light source and the photographic plate. In order to secure a properly timed spark of suitable char acter there must be available a means of generating electrical energy at very high voltage, a means of storing such energy, ap paratus for the regulation of the voltage at the proper time. The manner in which these components are associated is shown in the accompanying diagram.
The operation of the apparatus, which is all set up in a dark room, is as follows : Having put the film or plate in place, the operator sets the spark switch and starts the generator. The con densers, when charged, are automatically disconnected from the generator and a signal light is turned on as soon as the potential across them reaches the magnitude at which the regulator is set to function. The gun is then fired immediately. When the sound wave from the bullet or the propelling charge, according to cir cumstances, impinges upon the diaphragm of the interrupter, the electric circuit to the spark switch is momentarily opened. The switch arm then closes the so-called trigger gap at such time as the bullet or shot being photographed has arrived in front of the photographic plate. As the trigger gap is closed, the condensers discharge through the trigger gap, spark gap and damping resis tance which are all connected in series. If the spark lasts while the bullet moves appreciably blurring will result. Since the spark discharge from a condenser is not of itself of the proper char acter, the so-called damping resistance is introduced in the spark circuit. In this way a single spark of great brilliance and lasting
for only one or two millionths of a second is obtained.
Some idea of the accuracy of the equipment may be obtained from the fact that as many as 8o consecutive shot shells have been photographed in one lot without a single failure.
Applications to Ballistics.—There is, perhaps, no question in the realm of the ballistics of small arms upon which there has been so great a diversity of opinion as that of acceleration of a projectile after leaving the muzzle of the gun; this effect has been supposed by some authorities to exist "at points from 20 to 30 ft. in advance of the muzzle." Plate II., fig. 3, shows the bullet of a .3o calibre Springfield rifle. In this photograph the bullet is out I I in. from the muzzle and yet it has already out-distanced the outermost effect of the propelling charge.
No bullet is absolutely gas tight and figures show the gas which has leaked past the bullet while still in the barrel. Plate I., fig. 5, shows the .45 calibre Colt automatic pistol. Similar photographs show that no upward tilting of the muzzle occurs until the bullet is several feet from the muzzle. That is to say the so-called kick in no way affects the accuracy. Plate I., fig. 3 shows a modified .3o calibre bullet after passing through a soap bubble filled with a mixture of hydrogen and air which produces the distortion in the sound waves shown. Plate I., fig. 4 shows a steel ball after falling into a soap bubble which closed after it. Note small bubble between ball and top of film. Plate II., fig. 4 and 5 shows the muzzle of a .45 calibre Thompson sub-machine gun with the Cutt's compensator attached. (P. Q.) SPARK or SPARKING-PLUG: see INTERNAL COMBUS TION ENGINES and MOTOR CAR.