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condenser. Our standard voltage is of the order of 20 kv. According to the desired light output, the condenser capacity is fixed between 0.001 and 0.1 /if, thus yielding an electrical energy between 0.2 and 20 j. The desired performance determines whether the units are equipped with ordinary paper capacitors made by the Societe Industrielle des Condensateurs, or with more expensive paper capacitors produced by Philips, especially for use with flash sources. For an energy of 2 j corresponding to a capacity of 0.01 /if the natural frequency of the discharge circuit is 1 me for the ordinary type, while with the special type the discharge is almost aperiodic and the voltage falls to 1% of its initial value within 0.1 /zsec. The discharge can be made fully aperiodic if a 5-ohm damping resistor is inserted into the circuit.
Whereas, as mentioned before, no marked progress has been made as far as exposure time is concerned, the advent of electronic triggering methods has made it possible to improve the timing accuracy by two or three orders of magnitude, thus eliminating one of the greatest difficulties of earlier methods of ballistic photography.
Fig. 4. Superimposed shadowgraphs of five 8-mm rifle bullets with photoelectric triggering. The photographic plate was shifted vertically after each shot. Triggering time-lag: 1 /zsec ±0.1 /isec.
Figure 3 shows the circuit arrangement of our standard unit. The 20-kv generator consists of a thyratron relaxation oscillator, an automobile ignition coil and a rectifying diode. The high voltage may be varied by changing the thyratron grid bias and, hence, the recurrence frequency. Triggering consists in applying a 15-kv negative pulse to the spark-gap control electrode. This triggering pulse is generated by a thyratron and a transformer. In accordance with the required timing accuracy, the latter is either an auto coil or a pulse transformer with a rise time of 0.1 /isec. The overall time-lag between the application of the timing pulse to the thyratron grid and the flash is of the order of 1 /tsec ±0.1 /isec with the pulse transformer and 10 times as much with the automobile coil. This timing accuracy is illustrated in Fig. 4, which shows five 8-mm rifle bullets recorded on the same photographic plate shifted vertically after each shot.
A complete unit, with a paper condenser and a triggering auto coil, may be seen in Fig. 5, with both covers removed. The 25 X 35 X 25 cm cabinet houses the power supply, the high
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May 1953 Journal of the SMPTE Vol.60