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Fig. 27. Eighteen frames taken at a rate of approximately 100,000 frames/sec and showing penetration of a rifle bullet through a glass plate.
flashes are converted into electrical pulses by means of a photomultiplier and a wide-band amplifier and appear on the oscillogram as interruptions of the trace. The instrument has an accuracy of 10~8 sec.
Electronic Spark Generator for Silhouette Cinematography
Although it stands in a class apart from those discussed above, this instrument is described here because it shows how electronic methods of control make it possible to modify the properties of a spark-gap.
The older Schatte10'11 and Toepler methods of generating spark sequences at high frequencies in a single spark-gap essentially consist in the gradual discharge of a large storage condenser into a small spark condenser by means, respectively, of a simple RC or LC
relaxation circuit. Such purely electrical control methods have two major disadvantages. First, the spark frequency is not constant throughout the discharge, but decreases in the Schatte method and goes through a maximum in the Toepler method. Also, the electric current through the spark-gap is never zero even during a portion of the recurrence period. This results in bad de-ionization conditions and the necessity of a powerful air blast for frequencies above 20 kc. These two difficulties are obviated in the electronic spark generator, designed by Paul Devaux, by controlling the charging current through the use of a vacuum tube.
The circuit diagram is shown in Fig. 28. The energy of the whole spark sequence is stored in the 1 /if storage condenser K, which is discharged in steps into the 0.001 /if spark condenser
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May 1953 Journal of the SMPTE Vol. 60