INDUCTANCE AND CAPACITY. 



365 



Dielectric strength. When the electromotive force E acting 

 to charge two bodies A and B, Fig. 3, is increased, a value is 

 eventually reached which breaks down, 

 or ruptures, the dielectric and allows 

 the charge on the bodies to pass in 

 the form of an electric spark. When 

 the bodies A and B are flat, as 

 shown in Fig. 4, the electromotive 

 force required to break down the in- 

 tervening dielectric is, for a given di- 

 electric, proportional to the distance 

 x between the plates, and the quo- 

 tient EJX is called the electric strength 

 of the dielectric. The following table gives the strengths of 

 various dielectrics. 



TABLE. 



DIELECTRIC STRENGTHS. 



Substance. 



Strengths 

 in Volts per 

 Centimeter. 



Oil of turpentine .... . . 94,000 



Paraffine oil 87,000 



Olive oil 82,000 



Paraffine (melted) 56,000 



Kerosene oil 50,000 



Paraffine (solid) 130,000 



Strengths 



Substance. in Volts per 



Centimeter. 



Paraffined paper 360,000 



Beeswaxed paper 540,000 



Air (thickness 5 cm.) 23,800 



CO 2 " " . . . . 22,700 



O " " 22,200 



H " " 15,100 



10. The condenser.* Electrostatic capacity. The amount of 

 charge q which flows out of B into A, Fig. 3, when the battery 

 is connected, is proportional to the electromotive force of the 

 battery. Therefore we may write 



q=CE (II) 



in which q is the charge that is drawn out of B and forced into 

 A in Fig. 3 by a battery of which the electromotive force is E, 

 and C is a constant depending upon the size and shape of A and 

 B, and upon the intervening dielectric. This quantity C is called 

 the electrostatic capacity or simply the capacity of the pair of 



* See pages 19 to 23 for a description of the Hydraulic Analogue of the Condenser. 



