Piezo-electric Property of Quartz, 



339 



and of magnitude equal to s/ 3 times the magnitude of each of 

 them, as is easily proved by the elementary geometry of strain. 

 Hence an elongation in the direction K'OK does not produce 

 zero of electric effect. In fact, there are no " Axen fehlender 



Kff. 4. 



Piezo-electricitat." The three axes so-called by Rontgen * 

 are the lines of elongation in Curie's experiment. Without 

 sufficient consideration it might be imagined that the six lines 

 corresponding to K'OK in fig. 4 are " Axen fehlender Piezo- 

 electricitat." On the contrary, elongation in the line K'OK 

 produces an electro -polarization which is the resultant of the 

 equal polarizations indicated by aa and bb, and which, as it 

 bisects the angle bOa, is in a line inclined at 45° to O'K the line 



* Wiedemann's Annalen, 1883, xviii. p. 215, 



