PYROELECTRICITY AND PIEZOELECTRICITY 153 



corresponds to the intensity of magnetisation of a magnet. We 

 should expect, on the theory, that if a pyroelectric crystal is 

 broken across an electric axis, then the two broken axes would 

 show apparent electrifications equal and opposite. Canton is 

 sometimes supposed to have shown this,* but he does not appear 

 to have gone further than a demonstration that each fragment 



4- 



FIG. 101. 



behaved with change of temperature like the whole crystal. The 

 experiment was first successfully made by Voigt.f 



Voigt's experiment on a broken crystal. He fastened 

 the two ends of a small tourmaline rod to two brass rods which 

 were connected electrically. He then broke the tourmaline in the 

 middle and dipped the two broken surfaces into mercury 

 cups connected to a Nernst-Dolaxalek electrometer which showed 

 charges on the ends. He found as a mean result that there was an 

 intensity of electrification at 24 C. of 33*4 E.S. units, or the 

 electrical moment of 1 c.c. at 24 C. was 33'4, the sign being 

 negative because the electrification was opposite to that which 

 would have appeared on the same surface with rise of temperature. 



Now for this kind of crystal the electrification gathering per 1 C. 

 rise of temperature was found to be about -f 1'2 E.S. units per 



33*4 



square centimetre. Hence at 24 H =-^- = 5% C., the electrical 



moment of the crystal should be zero. Or, if we return to the 

 molecular models, it may be that both effects, dilatation and inter- 

 molecular separation, exist, and that at 52 C. they balance each 

 other. 



Piezoelectricity: The discovery by the brothers 

 Curie. The discovery of piezoelectricity was made by the 

 brothers Curie by an experiment represented diagrammatically in 

 Fig. 102. C is a crystal hemihedral with inclined faces cut into 

 the form of a rectangular block. It was placed between the jaws 

 J J J J of a vice. Between the jaws and the crystal were ebonite 

 plates ee and tinfoil 1 1, the tinfoil being against the ends of the 

 crystal. The tinfoil was connected to the quadrants of an electro- 

 meter, and one tinfoil and the connected quadrants could be 

 earthed. 



* Kelvin's Hultimore Lectures, p. 560. Cf. Priestley on Electricity, 4th ed. 

 p. 300. 



t Wied. Ann., 60, p. 368 (1896). 



