ceramic magnets, "Mobilarc Gaps, " manufactured by the 

 Westinghouse Electric Corporation for protecting high- 

 tension power lines from lightning discharges. These 

 magnets spun the arc and thus avoided burning a gap at one 

 spot. The underwater spark gap which was designed around 

 the ceramic magnet is shown in figures 16A-B. While the 

 gap performed well, its over-all improvement over other 

 designs was questionable. In using the large circular gaps 

 it was difficult to produce a true "ring fire, " as the spark 

 tended to rotate around the gap in a random manner. 



EXPERIMENTS WITH RESONATORS 



A limited amount of work at obtaining spectral emphasis 

 in the 1000-c/s region by the use of water-filled resonant 

 chambers was described in reference 1. Several chambers 

 were designed and tested using these techniques. A typical 

 chamber is shown in figures 17A-B. The resonant frequency 

 was first calculated as previously described and checked 

 in water using Lissajous patterns. The agreement with 

 original calculations was very close (fig. 18). The chamber 

 was then shock excited into vibration by the underwater 

 spark, using electrical resonance in the charging circuit 

 (fig. 19). This combination worked very well. 



Dr. William Toulis, while at NEL, constructed a 

 compliant- tube type of resonant chamber (fig. 2 0) for 

 use with underwater spark gaps. Data from tests using 

 this device are shown in figure 21. 



II. THE PNEUMATIC SOUND SOURCE 



BACKGROUND 



In a further attempt to generate high-intensity sound 

 underwater, several simple experiments were performed 

 with the pneumatic sound source. Its principle of operation 

 is as follows: An elastic sphere is inflated underwater until 

 its expansion limit is exceeded and it ruptures. Upon being 



37 



