on certain of their surfaces , Conversely , when an electrical 

 field is established across these surfaces the crystal expands 

 or contracts in certain directions. The most comnon of these 

 materials are quartz s Rochelle salt, and ammonium dihydrogen 

 phosphate (NI^HgFf^o The first occurs naturally in certain 

 parts of the world, while the latter two may be grown syn- 

 thetically., 



The French physicist Langevin made the first sonar trans- 

 ducer by cementing a mosaic of quartz crystals between two 

 heavy steel plates, which also acted as electrodes, Langevin's 

 "sandwich" became the prototype for the very successful ASDIC 

 transducer developed by the British Admiralty,, and a similar 

 unit was developed for the U. S. Navy by the Naval Research 

 Laboratory 



Since most quartz is imported into the United States, an 

 early effort was made to design American transducers around 

 more available material. The success of the Brush Development 

 Company in growing large flawless crystals of Rochelle salt 

 early suggested the use of this material in place of the rarer 

 quartz, Rochelle salt, however , has certain disadvantages „ 

 It contains water of crystallisation, which makes it difficult 

 to handle and also seems to limit the amount of power which 

 may be applied to it. At about 55 degrees Centigrade, it 

 decomposes , 



The Brush Development Company Introduced a new piezo- 

 electric crystal which came to be called ADP (ammonium 

 dihydrogen phosphate), which has certain Important advantages., 

 It has no water of crystallization and decomposes at 190 de- 

 grees Centigrade, being useful up to about 130 degrees Cen- 

 tigrade., This material soon became available in sufficient 

 quantity to replace Rochelle salt, so that new, with few 

 exceptions, all new transducers are being designed around the 

 better properties of ADP, 



While the Harvard Underwater Sound Laboratory and the New 

 London Laboratory were working on magnetostriction units,, the 

 University of California Division of War Research at San Diego 

 worked on the development of piezo-electric transducers. The 

 Brush Development Company and the Bell Telephone Laboratories, 

 working principally under direct Navy contract, also did 

 notable work and freely exchanged information with the lab- 

 oratories working under NDRC, 



The UCDWR Transducer Laboratory, under the direction of 

 F, No Do Kurle, obtained the services of George Argabrite, an 



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