The Specific Inductive Capacity of Dielectrics. 40o 



stance, and by Maxwell the Dielectric Constant. If two 

 similar condensers are electrified from the same source they 

 will attain the same potential, and the charges they receive 

 will be proportional to their capacities. By determining the 

 ratio of these charges the ratio of the capacities is determined ; 

 and if air be one of the dielectrics, this ratio is the specific 

 inductive capacity of the other dielectric. This is the basis 

 of the majority of the methods for determining dielectric 

 constants. Again, the force of attraction exerted between 

 two plates charged to different potentials is proportional to 

 the specific inductive capacity of the dielectric separating 

 them ; so that, by weighing the force of attraction between 

 two charged plates the specinc inductive capacity of the inter- 

 vening dielectric can be determined. And finally, methods 

 have been employed based upon the fact that the period of 

 the electrical waves sent out when an electrical system is 

 discharged is proportional to the square root of the capacity 

 and self-induction of the electrical system. 



The method used probably more than any other is a modi- 

 fication of Thomson's induction balance first employed by 

 Gordon*. It consists of five parallel disks, three forming a 

 primary system and two a secondary. The primary system 

 consists of the two outside plates connected together, and the 

 middle plate. When a difference of potential is established 

 between the outside plates and the middle plate, the other 

 two smaller intermediate plates can be so placed in the 

 resulting field of force that there will be no potential -difference 

 induced upon them. The introduction of a dielectric between 

 any two of the plates will disturb the equilibrium, and in 

 order to again bring the small plates to equal potential, one or 

 both must be moved with regard to the primary system of 

 large plates. The amount of the change of position of the 

 small plates required to again secure equilibrium after the 

 introduction of the dielectric gives the means of computing 

 the change of capacity of the system produced by the intro- 

 duction of the dielectric, and hence the specific inductive 

 capacity of the dielectric. A successful modification of this 

 method has been used by Blondlot f , who, instead of securing 

 equilibrium by moving the secondary plate, introduced 

 between one of the primary and one of the secondary plates 

 such a thickness of another dielectric of known specific in- 

 ductive capacity that the spark ceased in a micrometer air- 

 gap placed between the secondary plates. In Blondlot's 

 experiment he used two thin wedges of sulphur arranged like 



* Phil. Trans. Koy. Soc. 1879, part 1, p. 417. 

 f C. R. cxii. p. 1058 (1891). 



