ELECTRICAL COMPRESSION OF TOURMALINE. 639 



Let us suppose that the bases of a prism of tourmaline are 

 covered with plates of metal A and B, one of which B is 

 connected with the earth, -while the other can be connected with 

 a source of constant potential. 



The pressure p and the potential x of the armature A, may 

 thus be varied by making the system pass through a closed cycle, 

 and we may take these two quantities as independent variables. 

 The quantity dm of electricity which the plate A receives is 

 expressed by 



dm = Cdx + hdp. 



The coefficient C is the capacity of the armature A at constant 

 pressure, and h is a negative coefficient if the end A of the crystal 

 is positively electrified by compression. The principle of the 

 conservation of electricity gives then 



(25) 



If / is the length of the crystal, we may also form the 

 equation 



dl = adx + bdp , 



in which b is the coefficient of elasticity of the crystal. 



If we apply the principle of the conservation of energy as 

 we have done above for M. Righi's tube, we deduce from it 



a= -h. 



As the value of h is negative, equation (26) shows that a 

 tourmaline becomes longer, when it is electrified in the same way 

 as it would be by a rise of temperature, and this elongation is 

 proportional to the potential. A change of structure results, and, 

 no doubt, also an alteration of optical properties analogous to 

 that observed by Dr. Kerr in transparent bodies. 



The factor h is constant, since, according to MM. Curie, the 

 electrification is proportional to the pressure ; hence the capacity 

 of a condenser with a plate of tourmaline is independent of the 

 compression to which the crystal is subjected. 



655. GENERALISATION OF LENZ'S LAW. In all the preceding 

 cases the converse of the phenomenon, the existence of which is 



