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-MOOO V
+300 v
V
Delayed pulse
Fig. 13. Simple RC delay circuit.
coupled multivibrator and the phantastron, could have been used. But if the flash is to be delayed with respect to a previous flash — as is the case, for example, in multiple-flash photography — it is extremely difficult to make such vacuum-tube delay circuits insensitive to the high-voltage discharge of the first flash. Our purpose being the development of an all-purpose reliable delay system, we decided to use the very simple RC circuit, the principle of which is shown in Fig. 13.
When the primary pulse fires thyratron TI, condenser C, initially charged at 600 v, discharges through resistor R and the plate-cathode space of TI. The potential VA at point A, initially — 300 v, then tends exponentially toward +300 v, with the time-constant RC. Thyratron T2 fires and thus generates the delayed pulse when this exponential voltage wave goes through its firing voltage which, for the sake of clarity, has been assumed negligible. The values of R and C should be so chosen that the internal resistance of thyratron TI, while conducting, is negligible with respect to R. If the ratio of maximum to
minimum delay is kept at a reasonable value, say 20, the accuracy of this circuit is of the order of 0.5%, while its shortterm reproducibility is only a small fraction of 1%. This kind of performance is adequate for most practical applications.
Kerr-Cell Shutter
The principle of the Kerr-cell highspeed shutter is too well known to be repeated here. Suffice it to say that certain fluids, such as nitrobenzene, become birefractive under the influence of an electrostatic field. A Kerr cell thus consists of a vessel filled with nitrobenzene and provided with a system of electrodes capable of creating in the liquid a uniform electrostatic field. If this system is placed between two crossed polaroids along the path of a quasi-parallel beam of light, and no voltage is applied across the electrodes, it lets through only a very small residual fraction of the incident light. It becomes transparent if a suitable voltage is applied across the electrodes. A high-speed shutter is obtained by combining a Kerr cell with a suitable control circuit generating at the
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May 1953 Journal of the SMPTE Vol. 60