49 2 MEASUREMENT OF CAPACITY. DIELECTRICS. 



quadrants communicates with one of the tinfoils, and with a standard 

 condenser of capacity C formed of concentric cylinders. The second 

 tinfoil is connected with the earth. The pressure P is then regulated 

 by variable weights, until the needle of the electrometer which is at 

 first deflected by the DanielPs element, reverts to zero. If D is the 

 electromotive force of a Daniell, and c the capacity of the system 

 formed by the tinfoil, the conductors, and the corresponding pair of 

 quadrants, the value of the quantity of electricity is ;;/ = (C + c] D. 



If then the standard of capacity C is removed, the pressure P' 

 necessary to produce equilibrium is again determined ; the quantity 

 of electricity is then m = cD. The difference P - P' of the pressure 

 is then capable of producing a quantity of electricity 



It is thus found that a pressure of i kilogramme exerted along 

 the axis of the crystal of tourmaline liberates a quantity of electricity 

 equal to 0*053 C.G.S. units; the same pressure applied along the 

 electrical axis of quartz gives 0*063 units. 



1083. The electrical properties which crystals acquire by com- 

 pression lead to a remarkable consequence relative to the changes 

 of dimensions which they should experience under the action of 

 electricity. For a parallelopipedon of quartz having two faces per- 

 pendicular to the optical axis, and two other faces perpendicular 

 to an electrical axis, the establishment of a difference of potential 

 between the latter should produce either expansion along the elec- 

 trical axes and contraction in the direction perpendicular to the optic 

 and electrical axes, or the reverse effects, according to the sign of the 

 difference of potential, the length parallel to the optical axis being 

 unchanged. This curious experiment has been made by MM. Curie 

 by using the difference of potential of a battery charged by a Holtz 

 machine. The expansions observed were found to be very nearly 

 equal to that shown by calculation. 



1084. There is a close analogy between the charge of a con- 

 denser and the polarization of electrodes (252). Any quantity m 

 of electricity which passes through a voltameter, without giving rise 

 to any apparent disengagement of gas, produces a difference of 

 potentials e between the electrodes ; if we put 



the factor c may be defined as the capacity of polarization of the 



