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Fig. 5. Spark generator for single shadowgraphic exposures, with covers removed.
voltage generator and the triggering circuit. The projector unit, mounted on a stand, contains the spark-gap and the spark condenser. The only noncommercial component is the heater transformer for the rectifying diode, which is insulated to ground for 20 kv. Several dozen units of this kind have been constructed at an approximate cost of $700 each.
In the units now under production, the relaxation high-voltage generator has been replaced by the standard high-frequency 22-kv generator made by Philips for television applications.
Before leaving this subject, it may be of interest to say a few words about the high-accuracy triggering method used before pulse transformers were available, which may still be useful for laboratory work. In this method (Fig. 6) the triggering thyratron plate was capacitancecoupled to the spark-gap trigger electrode. The latter was insulated from the cathode by an annular air gap. With its 2.5-kv plate voltage, the thyratron life was limited to about 300 flashes, which was adequate if none of them was wasted during adjustments. The flashes needed for adjusting the optical setup
were initiated by short-circuiting the thyratron by means of a relay-controlled mechanical switch. The main disadvantage of this triggering method was the rather critical nature of its adjustment.
For most applications the timing puke is derived from the observed phenomenon by means of a suitable transducer, e.g., a photoelectric cell, a crystal microphone, a solenoid, etc. If necessary, it is delayed an appropriate period before being applied to the triggering thyratron control grid (see below).
The unit may be used for schlieren and reflected-light photography if a suitable light source is substituted for the pinpoint spark-gap. The krypton tubes C and D of Fig. 7, through which 2 j may be dissipated in a few tenths of a microsecond, are particularly useful for schlieren photography, because the discharge takes place in a narrow cylindrical channel. Xenon tubes A and B withstand a 20-j discharge and yield a light output sufficient to photograph a 60 sq ft surface from a distance of 10 ft with an //number of 3.5. These tubes, whose breakdown voltages are relatively low, are normally mounted in series with a triggering spark
Fayolle and Naslin: High-Speed Photography
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