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Light jources
Fig. 24. V-setup for schlieren photography.
Their frequency range is covered by having one set of inductances for each required rate. These inductances are far from small and their storage is liable to become quite a problem. This difficulty is, of course, eliminated by the use of electronic circuits with radio potentiometers as variable elements. The first instrument of this type was developed during the last war by C. A. Adams at the British Armament Research Department.
There are two fundamental methods of triggering the elementary discharge circuits in succession at the desired rate :
1. By means of an electrical pulse delayed by an adjustable amount with respect to a master pulse produced by the phenomenon being studied. This system is effectively adopted in the Adams instrument.
2. By an electrical pulse delayed with respect to the previous discharge. This is the method used in the Cranz-Schardin circuit and in our own instrument.
Both methods have their advantages and drawbacks. If a large frequency range is to be covered, the first method is liable to introduce overlaps and interchange of frames at the higher frequencies, because of the unavoidable dispersion of the delay circuits. On the other hand, the minimum feasible delay sets a higher limit to the frequency
obtainable with the second method. This limit may be lower than that obtainable with the first method, which depends on the accuracy of the delay circuits.
Therefore, we would recommend the use of the first method for a relatively narrow frequency range up to a few megacycles, and that of the second method for a wider frequency range — say 20 to 1 — up to the maximum frequency set by the minimum delay.
In the case of our circuit, which involves two 2D21 thyratrons per stage, the minimum delay is of the order of 1 ^isec, so that the maximum frequency is 1 me. The delay circuits are set for a range of 20 to 1 .
Each of the 30 discharge circuits is triggered by the previous one, except for the first one, which is triggered by a suitable transducer actuated by the observed phenomenon. Use is made of the discharge and delay circuits described above. Figure 25 shows one of the discharge circuits and its coupling with the previous one. The timing pulse is derived from the previous discharge circuit by transformer action, by means of a few turns of wire wound round the high-voltage condenser lead. From then on, condenser C discharges exponentially through resistor R and thyratron Thi. Thyratron Th2 eventually
Fayolle and Naslin: High-Speed Photography
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