204 MEASUREMENT OF PRESSURES 



B. Cathode ray tube circuits. The most important requirements of 

 the cathode ray tube and its associated circuits are that the accelerating 

 voltages be high enough to provide adequate brilliance of the trace for 

 photography, and that these voltages be sufficiently well regulated to 

 prevent changes in sensitivity and undesired modulations of the light 

 intensity. For trials with service weapons, overall accelerating poten- 

 tials of 2,000 volts are usually adequate, while for small charge work on 

 shock waves potentials as high as 5,000 volts may be necessary. 



A factor in quantitative use of cathode ray tubes often not appre- 

 ciated is the fact that the deflection sensitivity of the tube varies in- 

 versely as the applied accelerating voltage. This voltage must, there- 

 fore, be kept constant to at least the same degree as the desired precision 

 of measurement; this regulation is accomplished by use of sufficiently 

 steady a-c supply voltage or regulation of the high voltage supply. 

 High voltage regulation is of advantage also in preventing supply volt- 

 age modulation of the light intensity, which otherwise frequently results 

 when the cathode ray tube grid is used to switch the beam on and off in 

 transient recording. This switching, almost always necessary if an 

 electrical time base is used and often so with moving film recording, is 

 described below. 



C. Ti7ne resolution and synchronization. In some respects, the 

 simplest way of obtaining time resolution of a transient signal is simply 

 to use a high-speed continuous film movie camera started manually just 

 before the occurrence of the transient. Electrical problems of syn- 

 chronization disappear and a virtually unlimited time duration is ob- 

 tainable, and these advantages may be sufficiently great to outweigh 

 the problem of handling and wasting large amounts of film. Usually, 

 however, this is not the case, and other means are preferable. One 

 method is deflection of the cathode ray tube trace by a linear time base, 

 or single sweep, circuit initiated electrically just before the occurrence 

 of the transient. The requirements of the circuit are linear deflection 

 with time, reliable triggering by readily available synchronizing pulses, 

 and a means of turning the tube beam on and off at the beginning and 

 end of the deflection. The circuits used for the purpose are, for the 

 most part, evolutions from standard designs. This method of obtain- 

 ing a time scale is best for relatively fast transients where most efficient 

 use must be made of the available light output from the cathode ray 

 tube, but it gives only a limited recording time for given time resolu- 

 tion. 



A useful compromise between use of a movie camera and of electrical 

 trace displacements is the rotating drum camera, in which, as the name 

 implies, a short length of 35 mm. film is attached to a drum rotating on 

 an axis parallel to the displacement of the cathode tube trace by the 

 signal. Drums with diameters of 3 or 4 inches driven at speeds up to 



