448 



Tables 529 and 530 

 ELECTROSTRICTION. PIEZO-ELECTRICITY 



Electrostriction is a phenomenon observed when an isotropic dielectric is placed in an 

 electrostatic field (F), the form and volume of the dielectric altering. Similar effects 

 occur in anisotropic materials but are obscured by piezo-electric effects. Piezo-electricity 

 occurs when a crystal dielectric is mechanically strained becoming electrically polarized. 

 The magnitude and direction of the polarization (P) depends on the crystal used, the 

 amount of strain and its direction relative to the axes of the crystal. Pyro-electricity is of 

 the nature of a temperature-coefficient dp/dt. For fuller discussion and more extensive 

 data see I.C.T., 6, 207, 1918 (Cody), 



TABLE 529. — Electrostriction (Means) 



Glass Paraffin Ebonite Rubber (vulcanized) 



A1/1E 2 .4 X io n 00 X io" 12 600 X I0- 12 6000 X 10- 12 cmVcg.s.e 2 



These values divided by 1.11 X 10" 5 for values in cm 2 /volt 2 . 



TABLE 530. — Piezo-electricity 



Addenda to Table 528, p. 447, Dielectric Constant of Rochelle Salt: 



The polarization of the Rochelle salt dielectric in an electric field is somewhat analagous 

 to the behavior of the magnetization of iron in a magnetic field, showing both saturation 

 and hysteresis. The dielectric constant D depends on the initial and final fields and the 

 hysteresis. 



Initial field, 765 v/cm ; Final field, 690 v/cm ; Average D (23° C), 40 



765 —153 205 



765 -765 157 



880 86 



The last value may be fair value for ordinary purposes. The electrodes were tinfoil at- 

 tached with shellac. The field was applied perpendicular to the a axis. Like piezo-electric 

 properties, the dielectric constant varies with different crystals. It depends on the tempera- 

 ture as follows: (field to 880 v/cm) 



— 70° C, D = 12 ; — 40 , 14 ; — 20 , 48 ; 0°, 174 ; -f 20 , 

 (Data from Valesek, University of Minnesota, 1921.) 

 Smithsonian Tables 



; +30 , 52. 



