262 SHOCK WAVE MEASUREMENTS 



Whether and to what extent predicted negative pressures, or ten- 

 sions, are actually realized depends upon the experimental conditions. 

 Various observers have succeeded in obtaining absolute tensions of as 

 high as several hundred pounds per square inch. Values of this magni- 

 tude are obtained only for clean, air-free water, however, and under 

 more usual explosion conditions it seems improbable that open water 

 can sustain tensions greater than one atmosphere at most. The evi- 

 dence for this belief is partly based on the estimate that a tension as 

 low as this value will support bubbles of diameter as small as one or two 

 microns in diameter, and partly based on the formation of cavitation 

 bubbles in regions of water behind a rarefaction in which tensions of this 

 order were expected. It is therefore reasonable to suppose that water 



TIME 



Fig. 7.21 Gauge pressure as a result of surface reflection. 



not especially purified is incapable of supporting negative pressures of 

 more than an atmosphere, and possibly much less. Instead, a region of 

 cavitation containing many small bubbles is formed which prevents the 

 increase in negative pressure. 



Although the existence of small negative pressures in water is not 

 ruled out, the occurrence of tensions at an interface or boundary is very 

 much less likely, owing to the effects of oil or dirt on such boundaries. 

 For example, gauge measurements of pressures resulting from surface 

 reflections indicate within experimental error negative pressiu'es equal 

 to the hydrostatic pressure at most, as indicated in Fig. 7.21. The 

 absolute pressure is therefore zero after arrival of the siuface reflection, 

 and the effect of the surface is to cut off the pressure wave. The im- 

 pulse and energy in the resultant wave are therefore decreased below 

 their value in free water, the effect increasing with distance from the 

 source and at points closer to the surface. This "cutoff" effect is of 

 importance both in measurement and in predicting the effect of charges 

 filed near the surface. It is also sometimes useful in locating the posi- 



