ELECTRIC CHARGE. THE CONDENSER. 175 



cient to rupture it. Thus, air at ordinary atmospheric pressure 

 is ruptured by an electric field of which the intensity is about 

 24,000 volts per centimeter, and kerosene is ruptured by an elec- 

 tric field of which the intensity is about 50,000 volts per centi- 

 meter. The dielectric strength of a substance varies greatly with 

 its degree of purity. 



When an insulating substance is placed between two flat metal 

 plates A and B, as shown in Fig. 'i 08, the substance is sub- 

 jected to a uniform electrical stress (uniform electric field) when 

 the plates are connected to an electrical machine, and the electro- 

 motive force required to rupture the substance is quite accurately* 

 proportional to the thickness of the insulating layer, provided the 

 insulating substance is homogeneous like air or oil ; or, in other 

 words, a fairly definite intensity of electric field (volts per centi- 

 meter) is required to rupture a homogeneous substance like air or 

 oil, and such a substance has therefore a fairly definite dielectric 

 strength. Most solid substances, however, are non-homogene- 

 ous. Thus, the rubber gum which is extensively used for insu- 

 lating wires is " filled " with finely divided clay and is therefore 

 non-homogeneous. Sheets of window glass are usually filled 

 with fine bubbles and are therefore non-homogeneous. Thick 

 sheets of vulcanized fiber are usually charged with moisture in 

 the interior and dry near the surface, and they are, therefore, non- 

 homogeneous. The electromotive force required to rupture a 

 non-homogeneous substance is not even approximately propor- 

 tional to the thickness of the layer, and iHs<therefore customary 

 to specify the dielectric strength of solid insulatmg substances by 

 giving the electromotive force required to rupture a specified 

 thickness. 



The least roughness of the surface of the metal plates A and 

 B, in Fig. 108, or particles of dust floating in the dielectric, pro- 

 duce great variations in the value of the electromotive force 

 required to rupture a dielectric. The action of these irregular- 

 ities of surface and of floating particles is shown somewhat exag- 



*See Art. 127. 



