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High-Speed Motion Picture Photography*
ARE TWO METHODS of taking high-speed motion pictures _ widely used in the United States today. Furthermore there is work being conducted at present which will combine both of these methods into a third method.
The first general group are those motion pictures made by the use of a continuously moving film camera and an associated trigger circuit which fires a high-voltage gas-discharge tube at a selected frequency. This particular method has been developed by Dr. Harold Ei* Edgerton and his associates at the Massachusetts Institute of Technology.
The first work done in this field was to use a gas-discharge lamp to secure single exposures with a view camera. The gas-discharge tube consists of either a glass or a quartz tube which is filled with a rare inert gas such as xenon and possibly krypton added to it. There is an electrode on both ends of the tube and a third electrode outside of the tube. A high-voltage direct-current potential of approximately 2000 volts, though the voltage may be considerably higher, is kept across the first two-mentioned electrodes. The gap is great enough so that breakdown does not occur. This high direct-current potential is stored in a capacitor having a capacity of from 0.3 microfarad to 1300 microfarads. When an induced high voltage (from 10,000 to 15,000 volts) is applied to the third electrode on the grounded side of the capacitor ionization of the gas takes place which lowers its resistance and permits the stored charge in the capacitor to discharge. This discharge through the inert gas causes a very brilliant flash of short duration. The capacitor and the external resistance of the circuit are the controlling factors of the actual time of the flash. In the first work done along this line a raw spark in air was fired in the same manner and using a smaller capacitor but of the same or higher voltage. Flashes occurring in from */% to 2 microseconds were easily obtained. The gas tubes as now generally used work from 1 to 2000 microseconds.
* Presented April 6, 1949, at the SMPE Convention in New York. 440 NOVEMBER, 1949 JOURNAL OF THE SMPE VOLUME 53