ELECTRIC CHARGE AND THE CONDENSER. 



127 



78. Inductivity of a dielectric. The capacity of a condenser 

 depends upon the size of the plates, upon the thickness of the 

 dielectric and upon the nature of the dielectric. The dependence 

 of the capacity of the condenser upon the nature of the dielectric 

 is a matter which must be determined purely by experiment. 

 Thus Fig. 85 represents two metal plates with air between them, 

 and Fig. 86 represents the same plates immersed in oil. The dis- 

 tance between the plates is understood to be the same in Figs. 85 

 and 86. Let C be the capacity of the condenser in Fig. 85 

 with air as the dielectric, and let C' be the capacity of the con- 

 denser in Fig. 86 with a given kind of oil as the dielectric. The 



wire 



air 



air 



wire 



B 



Fig. 85. 



Fig. 86. 



ratio C'jC is called the inductivity* of the oil. Thus the induc- 

 tivity of kerosene is about 2.04, that is, the capacity of a given 



TABLE. 



Inductivities of Various Substances. 



Crown glass (according to composition) 3.2 to 6.9 



Flint glass (according to composition) 6.6 to 9.9 



Hard rubber 2.08 to 3.01 



Sulphur (amorphous) 3.04 to 3.84 



Paraffin 2.00 to 2.32 



Shellac 2.74 to 3.67 



Ordinary rosin 2.48 to 3.67 



Mica (according to composition) 5.66 to 10 



Petroleum about 2.04 



Water about 90. 



* What is here called the inductivity of a dielectric is sometimes called dielectric 

 constant, or specific capacity of a dielectric, or specific inductive capacity of a dielectric. 



