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Variation of voltage Vc across spor* .condenser ond
-Q-rLrL grid volttt3«Vqi of control .tube
Fig. 28. Circuit diagram of spark generator for cinematography at 50,000 frames/sec.
C across the spark-gap S. The charging current is controlled by transmitter tube TI in such a manner that it is interrupted during the time interval /i/a preceding the spark breakdown, during which the voltage across S remains constant and equal to Vi. Thus the spark-gap breakdown voltage Vz should be between Vi and the voltage V across K, initially 10 kv. Tube TI is itself controlled by receiver tube T2, whose grid voltage is derived from the spark condenser voltage by means of a potentiometer and a bias battery. Just after the spark breakdown, TI is conducting while T2 is cut off. The spark condenser voltage and, hence, the grid voltage of T2, then increase exponentially. When T2 starts drawing plate current, the grid voltage of TI begins to decrease, until time ti when TI is completely cut off and the charging current is interrupted. From then on, the voltage across C remains constant until a rectangular pulse is applied to the grid of TI at
time /2. Charging of C is then resumed until time t3, when breakdown occurs. The voltage across C is again at zero at time t\, when the circuit is ready to repeat the same cycle of operation. The periodic pulses applied to the grid of TI are produced by a variablefrequency pulse generator exterior to the spark generator proper. Figure 29 shows the spark generator with the cover removed.
The above circuit makes it possible to reach a rate of 50,000 sparks/sec in air without any air-blast.
The images are recorded by means of the rotating-mirror camera shown in Fig. 30, with the optical setup of Fig. 31. The fixed rim of this camera holds about 2.5 m of standard 35mm film. At the maximum mirror speed of 20,000 rpm, about 200 circular images 10 mm in diameter may be recorded at the rate of 50,000 frames/sec. Figure 32 shows a portion of a typical record of the impact failure of a standard European notched
Fayolle and Naslin: High-Speed Photography