PYROELECTRICITY. 485 



A tourmaline, which is kept at a constant temperature for a long 

 time, seems absolutely neutral ; the electrical properties of which it 

 may be the seat are compensated by a superficial layer, which is 

 produced spontaneously by conductivity (619), or which has been 

 produced by connecting the surface with the earth for instance, by 

 passing the crystal through the flame. 



When the crystal is heated, one end becomes negative and the 

 other positive ; the reverse is the case if the crystal is cooled. We 

 may, with Riess and Rose,* call that end of the crystal, the electrical 

 sign of which is the same as that of the change of temperature, the 

 analogous pole ; and that end whose electrical sign is contrary to the 

 change of temperature, the antilogous pole. The axis of polarization 

 is the direction from the antilogous to the analogous pole. 



The ends of the tourmaline being covered with a conductor 

 tinfoil or wire being rolled round it, for instance Gaugain connects 

 one of them with the earth, and the other with a discharging electro- 

 meter ; he has thus found that 



i st. The quantities of electricity disengaged at the two poles are 

 equal and opposite for the same change of temperature, and are 

 independent of the velocity of heating or of cooling. 



2nd. The quantities of electricity disengaged at one end are 

 equal and of opposite sign for two changes of temperature which 

 are the inverse of each other. 



3rd. These quantities of electricity are proportional to the section 

 of the crystal, perpendicular to the optical axis, and independent of 

 its length. 



In order to explain these curious properties, Sir W. Thomsonf 

 assumes that the crystal is naturally in a state of electrical 

 polarization. 



A change of temperature, starting from what is apparently the 

 neutral state, produces a polarization equal to the difference of those 

 which correspond to two determinate temperatures / and t lt and the 

 crystal is comparable with a body magnetised uniformly. Canton 

 observed, in fact, that when an electrified tourmaline is broken, the 

 pieces have each two poles, and the faces corresponding to the 

 fracture are oppositely electrified. 



If we consider a cylindrical rod of tourmaline, terminated by 

 faces perpendicular to the axis, and insulated during the change 

 of temperature, the potential is positive on one half and negative 



* RIESS and ROSE. Archives de V Electricity Vol. in., p. 585. 

 f THOMSON. Phil. Mag. [5], Vol. v., p. 24. 1878. 



