Service experience also showed, however, that projectors of the 

 quartz type,, which require high voltages, were subject to break- 

 down due to the intrusion of moisture particularly in the in- 

 sulation of the wires o A quartz projector is, moreover, sharply 

 resonant and consequently can receive only a very small portion 

 of the spectrum of supersonic noise „ 



To overcome these disadvantages of the quartz projectors 

 experiments were conducted with another type of piezo-electric 

 crystal, Rochelle salto Equipments employing Rochelle salt 

 projectors were built and designated as JK for supersonic 

 listening and QB for echo-rangingo The results were gratify- 

 ing in that the search range was increased,, 



Rochelle salt s however, displayed certain shortcomings,, 

 While the crystals were very efficient, they were electrically 

 delicate and would burn out or melt if one tried to transmit 

 with too much power They also had to be protected from ex- 

 treme temperature such as is occasionally encountered under 

 the tropical sun. 



Magnetostriction , 



For this reason still another type of supersonic projector 

 was sought and the answer found in the phenomenon of magneto- 

 striction, according to which a ferro-magnetic material such as 

 nickel changes its length when magnetized and conversely changes 

 its magnetic state when strainedo Projectors involving the 

 magnetostriction principle were built into equipment of the QC 

 class for echo-ranging and the NM class for depth soundingo 



By 1933 , the first QC gear had been built and installed on 

 the new destroyers USS FARRAGUT and USS DEWEY„ Simultaneously, 

 the first standard QB installations were made on the new sub- 

 marines CACHLOT and CUTTLEFISH of the SS-170 class . Thus, the 

 beginning of standardized sonar gear coincided with the initia- 

 tion of the expanded Navy building program 



Characteristics of Sea Watf r 



It is worth noting that all of the improvements to this 

 earlier sonar pear succeeded in increasing by only a small 

 amount the range at which submarines can be detected by echo 

 meanso This is because the limitations are generally not in 

 the apparatus itself but in the sea water which carries the 



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