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establishments and proving grounds with modern experimental equipment in the fields of dynamic pressure recording, short-time measurement and high-speed photography. The nature of this task accounts for the special features of the instruments to be described below. In accordance with the views expressed by Dr. W. A. Wildhack,3 of the National Bureau of Standards, in his opening lecture to the Instruments and Measurements Conference held in Stockholm in 1949, we hold that "instrumentation has now become an art of its own." The development and use of scientific and technical instruments should therefore be handled by separate groups of workers. A corollary to this statement is that the instruments should be as easy as possible to manufacture, to handle and to maintain; hence their main features should be simplicity, ruggedness, reliability and flexibility. Also, and for obvious reasons, their cost should be as low as possible. It is clear that electronic instruments using as far as possible commercially available radio components should, if properly designed, fulfill all the above requirements.
Electronic Spark Generator for Single Shadowgraphic Exposures
This instrument was primarily developed for use with the shadowgraphic optical method using the Libessart pinpoint spark-gap, although, as will be mentioned later, it may also be used in
Fig. 1. Shadowgraphic exposure of an 8-mm rifle bullet in flight, with a velocity of approximately 1950 fps; flash duration at half maximum amplitude is 0.1 /xsec, effective time of exposure much shorter. The bullet is breaking a glass plate with a zig-zag metal coating, thus generating the timing pulse.
a schlieren setup or even for low-energy reflected-light exposures.
No particular effort was made concerning the discharge circuit proper. Since the early days of spark photography it has been a well-known fact that the combined use of a high-charging voltage, a small low-inductance spark condenser and short leads makes it possible to reduce the actual flash duration* to 10~7 sec or less. With operation under adequate optical conditions, the duration of the photographically active portion of the flash, as measured by the optical blur due to the motion of the subject, may be reduced to 10~8 sec. The picture quality that can be obtained is demonstrated in Fig. 1, which shows an 8-mm bullet in flight. The discharge circuit used, comprising a 0.004 /zf ceramic condenser produced by the United Insulator Co., a British firm, had a natural frequency of 30 me/sec. There would be no point in improving these performances any further, since other limitations are set by the finite dimensions of the pinpoint spark-gap, diffraction phenomena, grain size of the photographic material, etc.
Figure 2 shows our standard pinpoint spark-gap assembly, of the Libessart type. The steatite insert with a hole
* The flash duration is arbitrarily defined as the time during which the light-vs.time curve is above its half-maximum amplitude.
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May 1953 Journal of the SMPTE Vol. 60