MEASUREMENT OF PRESSURES 205 



3,600 RPM or more are commonly used, giving time resolutions on the 

 film up to 1 or 2 inches per millisecond. 



Either the electrical time base with fixed film or the drum camera 

 with moving film requires synchronization of the recording interval with 

 the events to be observed. One method is to use changes in the firing 

 current of the detonator cap initiating the explosion. Unless special 

 caps are used or large firing currents employed, the detonation of the 

 charge is usually delayed by several milliseconds after the circuit is 

 closed and the interval is often not sufficiently reproducible if accuracy 

 to a fraction of a millisecond is needed. The breaking of the firing cir- 

 cuit by the force of the explosion is also quite undependable, particu- 

 larly for large charges. A number of special firing circuits have been 

 devised which permit satisfactory operation, but none has been found 

 foolproof. Another expedient is to use a pressure operated switch 

 suitably placed near the explosion. The most satisfactory switch of 

 this kind is probably simply a piezoelectric "pilot" gauge designed pri- 

 marily for ruggedness, as little maintenance is required with a good 

 gauge and its operation is both simple and readily predictable. In these 

 methods or others, it is often desirable to delay the operation of the 

 various circuits to be synchronized by controllable intervals, and rela- 

 tively simple trigger circuits accomplish this purpose satisfactorily. 



D. Photograjphic recording. In photographic recording of transient 

 oscillograph traces there is seldom an excess of fight and it is usually 

 desirable to use the fastest available lenses of reasonably good quality, 

 the most generally employed type being an f/2.0, 50 mm. focal length 

 lens. The trace of either a single tube or a combination of several tubes 

 mounted near one another is recorded on the film. The type of film 

 used is not critical, although the best results have been obtained with 

 Eastman Fluorographic, and adequate densities for most purposes are 

 obtained without use of prefogging or intensification. A trace 1 milli- 

 second long on a 5 inch tube screen with a record of an exponential 

 pulse of time constant 50 microseconds can be photographed with good 

 readability using an f/2.0 lens and 2,200 volts overall accelerating po- 

 tential on the cathode ray tube. For fixed film recording using a time 

 base either 35 mm. film and a rebuilt miniature camera, or cut film with 

 a special holder to permit several exposures on a single film, have been 

 used. 



E. Calibration circuits. Any system for recording transients must 

 include accurate means of calibrating voltage sensitivity and the time 

 scale, and it is highly desirable to extend the meaning of sensitivity to 

 include the complete transient characteristics of the electrical systems. 

 A common practice in much oscillograph work is to calibrate both volt- 

 age and time scales at once by recording a sine wave of known ampli- 

 tude and frequency. Although this method is economical of time, it 



