198 MEASUREMENT OF PRESSURES 



One final possibility remains to be considered, that of using a bal- 

 anced, or push-pull, line. If the two sides of such a balanced line de- 

 velop equal cable signals, these signals will cancel each other exactly if 

 the rest of the system is arranged to respond only to potential dif- 

 ferences between the two sides. Hence the use of push-pull gauges and 

 amplifiers with a perfectly balanced line represents the ideal solution in 

 principle. In practice push-pull operation has usually been found to 

 reduce the signal considerably but never completely. The improve- 

 ment was found only if the two high potential conductors were sur- 

 rounded by a common dielectric and shield. The use of two separate 

 single conductor cables is, on the other hand, as likely as not to make 

 matters worse, an example of the nonreproducibility of the phenomenon. 

 The failure of two conductor cables ever to be perfectly symmetrical 

 means that a cable should be used in which the separate signals are 

 small. If this condition is satisfied, as it appears it would be wdth 

 graphite or conducting rubber layers between dielectric and shield, the 

 use of a balanced line should be the best ultimate solution for the cable 

 signal problem. That it has not been used more extensively up to the 

 present is explained largely by limitations of available equipment and 

 cable. 



5.9. Electrical Recording of Underwater Explosion Pressures 



The most powerful tool for recording of transient pressures is 

 undoubtedly the cathode ray oscillograph, and in many applications 

 where high time resolution is necessary it has no satisfactory substitute. 

 As a result, nearly all measurements of underwater pressure-time curves 

 involve its use. In such transient measurements, a number of factors 

 are involved in addition to those encountered in the more qualitative 

 uses of oscillographs. These added requirements are sufficiently severe 

 that, for the most part, present-day commercial equipment is inade- 

 quate and one must either modify such equipment considerably or de- 

 sign instruments specifically for the purpose. This section outlines the 

 more important requirements and some of the solutions which have 

 been developed.^^ 



The necessary elements of a transient recording system can be 

 grouped in five broad categories: (a) signal amplifiers, (b) cathode ray 

 tube indicator, (c) time resolution of the record and s^^nchronization, 



(d) photographic or other means of obtaining a permanent record, 



(e) calibration of the sensitivity and time resolution. The relation of 

 these functions is indicated schematically in Fig. 5.23. The signal 

 from the piezoelectric or other pickup is amplified by whatever factor 

 is necessary and applied to one pair of deflection plates of the cathode 



^° Designs of electronic e(iuipmcnt for such tran.sient measurements are described 

 in reports by Greenfield and Shapiro (42) and by Cole, Stacey, and Brown (21). 



