At about the same instant that the current jumps, or an infinitesimal fraction of a second later, the light of the resulting spark starts toward the camera at a trifle more than 186,000 miles a second. It is a race between the spark current and the spark light as to which arrives first. The current jumps just before the spark appears; so it is possible for the current to reach the camera and close the shutter even before the light which is to be pictured arrives.
By lengthening the wires between the spark gap and the camera the light is allowed to arrive first. By suitable adjustment of the wiring, the shutter can be made to close during any one-billionth of a second interval during the first four ten-millionths of a second of the spark's short life.
The camera shutter consists of two Nicol prisms of Iceland spar and balsam, arranged in such a way that under ordinary conditions the light coming from the spark is stopped by polarization and prevented from reaching the camera. Between these two prisms, however, is a solution of chemicals which will depolarize the light and allow it to continue.
The wires leading from the spark gap connect with this solution. When the current jumps across the gap it races down the corridor and electrifies the solution for about one-billionth of a second. This electrification removes the depolarizing effects of the solution and light passage stops; in other words, the shutter is closed.
"Good Lord! What's this?"