80 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



varies, within wide limits, with the temperature; also the resonance 

 frequency of the crystal changes with the temperature. 



The X cut of the rochelle salt crystal is primarily applicable for face- 

 shear motion. Crystals can also be cut for transverse (longitudinal) 

 compression-expansion, and the cuts can be combined to form bender 

 and twister bimorph systems. 



The volume resistivity is of the order of 10 12 ohm-cm or more. The 

 mechanical strength is considerably lower than that of quartz. 



Ammonium dihydrogen phosphate ("ADP," NH 4 H 2 P0 4 ) has a 

 higher thermal stability than rochelle salt; it is stable up to a tem- 

 perature of 180°C, but is used, for technical reasons, with an upper 

 limit of about 125°C. It operates satisfactorily in a range from zero 

 to 94 per cent relative humidity if condensation of water on the sur- 

 face (causing surface leakage) is avoided. The volume resistivity is 

 considerably lower than that of the other substances described above ; 

 it is about 10 10 ohm-cm and decreases strongly with increasing tem- 

 perature and content of impurities. 



The material (primarily the Z cut) is applicable for face-shear and 

 longitudinal expansion operation and for twister bimorphs. The 

 sensitivity is high. A crystal 1 cm thick furnishes for a load of 1 dyne/ 

 cm 2 an output voltage of 1.78 X 10~ 4 volt. 



Barium titanate ceramics (BaTi0 3 ). The single barium titanate 

 crystal is ferroelectric, i.e., if exposed to an electric field it becomes 

 polarized and maintains its polarization after the field is removed. 

 Application of a field in the reverse direction causes a reverse polari- 

 zation. The process follows a hysteresis curve. 



In the ceramic polycrystalline form, the individual crystals are 

 randomly oriented. By the application of a polarizing electric field 

 (of order of 1,000 to several thousand volts per millimeter for about 1 

 hr at room temperature), the ferroelectric domains are lined up, and 

 the material becomes piezoelectric. The material loses some of the 

 piezoelectric activity in the first few days and remains fairly constant 

 afterwards. The aging process can be accelerated by subjecting the 

 material to higher temperature (80°C). The addition of about 4 per 

 cent lead titanate increases the long-time stability. 



Barium titanates are applicable for thickness compression- 

 expansion and for transverse compression-expansion operation, 

 Figs. ( 1-2) 59a and b; they can be combined to form bender bimorphs, 

 but they do not operate in shear modes and, consequently, they 

 cannot be combined to form twister bimorphs. 



The thickness compression and the transverse compression effects 

 are opposite in sign but not equal in magnitude. A volume change 



