49 MEASUREMENT OF CAPACITY. DIELECTRICS. 



effect are parallel to the diagonals of the cube. Chalcopyrite shows the 

 same properties, without its being possible to determine the direction 

 of the axes. These two crystals also show hemihedry with inclined 

 faces. The multiplicity of axes in grey copper does not seem to 

 show that we are dealing with regular polarization or with a true 

 pyroelectricity. 



1081. PIEZOELECTRICITY. It has long been known that pressure 

 may electrify certain bodies. Surfaces are thus obtained which are 

 charged with opposite electricities ; but the electrification of the 

 compressed bodies remains after the pressure has ceased, and pre- 

 sents no character of polarity. The phenomenon appears to be a 

 particular case of electrification by contact. 



MM. J. and P. Curie* have discovered that mechanical changes 

 in form also produce in hemihedral crystals with inclined faces a 

 special polarization which they call piezoelectrical. 



Take, for instance, the case of tourmaline terminated by two 

 bases perpendicular to the axis. Tinfoil is applied on each face, 

 and the crystal is compressed in the direction of the axis. The 

 two tinfoils are charged with opposite electricities, and in the same 

 direction as if the crystal had been cooled or undergone a thermal 

 contraction (654). When the pressure is removed, the tinfoils, 

 passing through the neutral state, become electrified in the con- 

 trary direction that is to say, in the same manner as by heating. 

 By first stretching, and then letting go, effects are observed re- 

 sembling those produced by removing and putting on pressure. In 

 all cases electricity is disengaged on the bases only. 



The pressure may be applied laterally and the electricity collected 

 on the bases. The direction of electrification along the axis is the 

 same in both cases. 



Consider, in like manner, a rectangular quartz prism with two 

 faces parallel to the axis, and two others perpendicular to a lateral 

 axis of pyroelectricity, the tinfoils being on the faces perpendicular 

 to the electrical axis. A pressure parallel to the electrical axis 

 produces polarization, and the negative tinfoil corresponds to the 

 edge which supports the face of the ditrihedron. Polarization also 

 takes place, but in the opposite direction, with pressure perpen- 

 dicular to the plane passing through the electrical axis and the 

 optical axis. Pressure parallel to the optical axis produces no 

 electricity. 



* JACQUES and PIERRE CURIE. Comptes rendtis, Vol. xci., p. 294, 1880, 

 passim. Journal de Physique [2], Vol. I., p. 245. 



