unsaturated compounds of high molecular weight were tried 

 for the dielectric material. Some have proved far superior 

 to commercial epoxy resins. One of the latter, known by 

 the trade name of "Scotchcast No. 2" has been satisfactory 

 up to levels of around 3000 watt-seconds. 



In recent experiments using an energy level of 6000 

 watt-seconds, it was found necessary to abandon the dielec- 

 tric material and simply fire a well insulated brass rod 

 against a heavy bronze ground plate. This may be one of 

 the practical solutions to the problem. 



Geometry of the Gap 



Since one of the basic aims of the work was to obtain 

 significant output levels in the low-frequency region below 

 5000 c/s, many configurations of electrodes were tried. 

 The importance of the gap geometry became evident early. 

 This factor can be made to focus sound, can affect the size 

 and shape of the bubble, and it controls the duration of the 

 pulse. The many bubble photographs included in the report 

 illustrate its significance; note the strange, almost "square" 

 shapes of bubbles produced under some conditions. The 

 phenomena associated with the geometry of the gap are not 

 fully understood, and should be studied. 



THEORETICAL DISCUSSION 



Frequencies Generated by an Underwater Spark Discharge 



The underwater spark discharge is essentially broad- 

 band in character and similar to the discharge of a chemical 

 explosive except that it is more susceptible to energy peaking 

 at certain frequency bands. From an echo-ranging stand- 

 point, high acoustic energy in the region between 1000 and 

 3000 c/s is desirable. Through the use of techniques 

 reported here, peak acoustic powers greater than those 

 obtained from large costly sound projectors have already 

 been achieved. Since we are not interested in wasting 

 large amounts of power in the region above 15 kc/s, we have 

 designed underwater spark gaps that generate large, low- 

 frequency bubbles. The bubble pulse from an underwater 

 spark can be quite useful, although previous experimenters 

 have given little attention to this fact. The frequency of the 



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