CHAPTER XII 



PYROELECTRICITY AND PIEZOELECTRICITY 



Pyroelectricity Historical notes Analogous and antilogous pole? 

 Some methods of investigating pyroelectricity Gaugain's researches 

 Lord Kelvin's theory of pyroelectricity Voigt's experiment on a 

 broken crystal Piezoelectricity The discovery by the brothers Curie 

 The piezoelectric electrometer Voigt's theory connecting pyro- 

 electricity and piezoelectricity Electric deformation of crystals 

 Lippmann's theory Verification by the Curies Dielectric subjected to 

 uniform pressure Change of temperature of a pyroelectric crystal on 

 changing the potential. 



CERTAIN crystals initially showing no electrification develop, if 

 heated uniformly, opposite surface electrifications on opposite 

 surfaces. If they are cooled from a neutral condition the polarity 

 is reversed. The phenomenon is termed Pyroelectricity, and it \\.-is 

 observed first in the eighteenth century. If these crystals, \\ ithout 

 being heated, are subjected to pressure along the axis of electrifica- 

 tion which was observed when the temperature was changed, then 

 opposite electrifications develop at the ends of the axis. If they 

 are subjected to tensions the polarity is reversed. The phenomenon 

 is termed Piezoelectricity. It was discovered by J. and P. Curie 

 in 1880. They found that the electrification 

 under pressure was the same in sign as that 

 due to cooling, while that under tension was 

 the same in sign as that due to heating. 

 The two phenomena are evidently related to 

 each other. 



The crystals in which they are observed 

 are those known as " hemihedral with in- 

 clined faces." They have axes with the 

 faces at the two ends unlike, so that they 

 are unsymmetrical with regard to a plane at 

 right angles to the axis through its middle 

 point. As an example we may take tourma- 

 line, of which Fig. 96 represents one form, 

 the axis of electrification being as shown 

 by the arrow. We shall describe first the 

 phenomenon of pyroelectricity. 



148 



FIG. 



