Salt Water vs. Fresh Water 



Almost all the underwater spark experiments to date have 

 been made with fresh water. On occasion salt water was 

 substituted with no apparent difference except in gap spacing. 

 Distilled water was also tried and the gaps would not fire. 

 In most areas, fresh water contains many impurities and" 

 therefore has a sufficiently low resistance. In one exper- 

 iment the gap was opened to a point at which there was not 

 sufficient potential to cause the gap to fire. Increasing the 

 salinity of the solution caused the gap to fire as before. 



High-Speed Photography 



High-speed photography was used to study the phenomena 

 of the underwater spark and pneumatic sound sources. For 

 nearly all this work two Eastman Type 3 high-speed cameras 

 were used, photographing the action from two angles. Pic- 

 ture rates for the underwater spark shots averaged about 

 2800 per second and for the pneumatic explosions about 

 2000 per second. Thousands of feet of 16-mm motion pictures 

 were made, and stills from many of the sequences are shown 

 in this report. Figures 8A-B are pictures of bubble develop- 

 ment in the laboratory tank under various conditions. The 

 deep-water photographs appear in subsequent sections. 



(A) From reflector gap. 



Gap which produced the "square" bubbles. 



(B) "Square" bubbles illustrating effects of 

 geometry of gap on size and shape of bubbles. 



Figure 8. Sequences from high-speed motion pictures 

 of bubble development produced in laboratory tank under 

 various conditions. 



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