PYROELECTRICITY. 487 



Kundt* has described a very simple method of showing the 

 electrification of various plates, which consists in powdering the 

 crystal with the mixture of sulphur and red lead which is used for 

 producing Lichtenberg's figures. 



1077. Various observers have noticed, in certain crystals, such 

 as topaz, prehnite, axinite, and boracite, the existence of two, three, 

 or even four axes of pyroeletricity ; but Freidel has observed that 

 two different directions are never found to which the regular charac- 

 teristics of tourmaline can be assigned. This multiplication of 

 electrical axes is, in fact, contrary to the idea of polarization. Two 

 or three polarizations in different directions would be equivalent to a 

 single polarization in a definite direction, as with the magnetisation 

 of anisotropic bodies (391) ; in a homogeneous crystal, at a uniform 

 temperature, experiment can only show one resultant axis. 



Most of the experiments in which it has been supposed that 

 there are different axes have been on macled crystals. This is 

 particularly the case with topaz and prehnite, which belong to the 

 right rhombic prism. The lozenge-shaped tablets present three 

 poles, one analogous in the centre of the lozenge and two antilo- 

 gous at the ends of the shortest diagonal, so that there are two 

 opposite electrical axes ; but these plates are formed of two halves, 

 the orientation of which differs by 180, and the crystal has really 

 only one pyroelectrical axis. Nevertheless, the experimental veri- 

 fication is often difficult because the crystals are usually too small 

 and too confused. 



Scolezite presents the same characters and lends itself better 

 to experiments. 



In other circumstances the effects observed arose in great part 

 from the shape of the specimens employed. 



1078. In order to eliminate the source of error due to the 

 shape of the crystal, Friedelf uses a face with parallel faces cut 

 perpendicular to the direction of the hemihedral axes, which are 

 always axes of pyroelectricity. By a change of temperature one 

 of the faces becomes positive and the other negative. The plate 

 being placed on a conductor connected with the earth, if the upper 

 face is covered with tinfoil connected with the needle of an electro- 

 meter, a disengagement of electricity is observed which, for a given 

 change of temperature, is proportional to the extent of the surface. 

 If the plate is not perpendicular to the pyroelectrical axis, the 



* KUNDT. Sitzb. der Akad. der Wiss. zu Berlin, p. 421. 1883. 



f C. FRIEDEL. Bulletin de la Societe de Mineralogie, Vol. n., p. 31. 1879. 



