Refraction of Quartz produced by Electric Forces, 137 



Crystal I., marked end at the top. 

 Below + , above — ; same effect as horizontal compression. 

 Below — , above + ; „ vertical „ 



Experiment VI. Crystal II., marked end at the top. 



Below + , above — ; same effect as horizontal compression. 

 Below — , above + ; „ vertical „ 



The experiments were repeated at very different times and 

 under varied conditions : thus in the later experiments there 

 was often only one crystal in the flask filled with benzol, the 

 other, which served as compensator, being surrounded by air ; 

 sometimes plates of mica or other pieces of quartz were chosen 

 as compensator ; but the same results described above were 

 always obtained, and never any thing different. 



It is well known that a compressed glass plate behaves 

 optically as a negative crystal the principal axis of which 

 coincides with the direction of compression. Since quartz is 

 a positive crystal, the results obtained above may be expressed 

 by saying that the double refraction of the pieces of quartz 

 examined increased when the marked end of the secondary 

 axis was charged with positive electricity and the unmarked 

 end with negative, and that, on the contrary, the double 

 refraction diminishes when the marked end of the axis is 

 negative and the unmarked end positive. 



It was now further examined how these ends behave in 

 piezoelectric relationship. The experiment was made by 

 covering the end faces of the crystal with tinfoil and com- 

 pressing it in a screw-press between plates of ebonite in the 

 direction of its length : the one strip of tinfoil was connected 

 with a delicate Fechner's goldleaf electroscope, capable of 

 indicating the charge of the insulated pole of a Daniell's 

 element by a marked deflection, the other was connected with 

 earth. 



The experiment frequently repeated gave uniformly the 

 same result, that the marked end of the secondary axis of 

 both pieces of quartz became negatively electrified upon in- 

 crease of pressure, and positively electrified upon decrease of 

 pressure ; the unmarked end became respectively positively 

 or negatively electrified. 



We may therefore express the result of the electro-optical 

 investigation thus : The double refraction of quartz increases 

 if positive electricity is communicated to that end of a secon- 

 dary axis which, upon increase of pressure acting in the 

 direction of this secondary axis, becomes negatively electrified, 

 and if at the same time negative electrity is communicated 



Phil. Mag. S. 5. Vol. 15. No. 92. Feb. 1883. L 



