MEASUREMENT OF CAPACITY. DIELECTRICS. 



needle of which is electrified ; the plate B is connected with the 

 other pair of quadrants. 



The needle having been brought to zero, by putting the plate B to 

 earth for an instant, the deflection a is observed, either if a vacuum 

 is made, or if air is replaced by another gas. 



If V is the electromotive force of the battery, p and // the co- 

 efficients for the two gases, the difference of potential between the 

 two plates, which was first V, soon becomes V (123), for the 

 charge of the plate is virtually the same. The change 5V, indi- 

 cated by the electrometer, is equal to V ( i - -, J , which gives 



x SV 



As the electrometer must be .very sensitive for this kind of 

 observation, the potential V is determined by the deflection which 

 a single couple produces. 



If we take as unit the specific inductive power of vacuum, we 

 find a number greater than unity for all gases, but less for hydrogen 

 than for air. It follows that the variation 8V is negative when any 

 gas is exhausted, or when air is replaced by hydrogen. 



Professors Ayrton and Perry worked by Thomson's method 

 (1049), using two condensers, a standard air one, and a lamellar 

 one of twelve plates, between which any given gas could be intro- 

 duced, or a vacuum made. 



The determination of the specific inductive capacity of a di- 

 electric has great theoretical interest, owing to the relation estab- 

 lished by Maxwell's theory between this constant and the refractive 

 index (634). 



1075. PYROELECTRICITY. Pyroelectricity is scarcely manifested 

 in any other than bad conductors, and is naturally connected with the 

 question of specific inductive powers. Older observations showed 

 that a rod of tourmaline, when heated, acquires the property of 

 attracting light bodies at its two ends. These phenomena, which 

 have been called pyroelectrical, have given rise to a great number 

 of researches, and the experimental laws have been established by 

 Gaugain.* 



* GAUGAIN. Ann. de Chim. et de Phys. [3], Vol. LVII., p. 5. 1859. See 

 MASCART. Traite d'Electr. Stat., Vol. n., p. 494. 



