40 ELECTRICAL ENGINEERING 



ductor is found to be 



1 1 1V 



. . (67) 



and the capacity of the cable per centimeter length is 



28. Dielectric Strength. As the difference of potential be- 

 tween two insulated conductors is increased the intensity of the 

 electrostatic field between them increases and the energy stored 

 in it increases. At a certain point the stresses in the dielectric 

 become so great that a rupture occurs and a discharge takes 

 place between the electrodes and the energy disappears from the 

 field. 



If the dielectric is a gas or a liquid any effect of the discharge 

 is remedied by circulation of the dielectric. When a heavy dis- 

 charge takes place through oil or other similar material it may 

 become partially carbonized, and the carbon particles tend to 

 line up in the intense field between the electrodes and form a 

 conducting bridge. 



In the case of solid dielectrics a rupture occurs which destroys 

 the insulating properties of the dielectric. 



The difference of potential or electromotive force between 

 terminals at which the breakdown occurs depends on the dielec- 

 tric material, on the distance between electrodes and on the dis- 

 tribution of the electrostatic flux in the dielectric, that is, on the 

 shape of the electrodes. Breakdown occurs due to high electro- 

 static stresses in the medium or due to a steep potential gradient. 



The potential gradient is usually expressed in volts per inch 

 or volts per centimeter, but since even one centimeter of dielec- 

 tric requires a very high voltage to puncture it the dielectric 

 strength may be defined as the difference of potential in volts 

 required to cause a discharge through one millimeter thickness 

 of the material but the shapes of the electrodes should always be 

 specified. 



In the following table the approximate dielectric strengths of 

 various insulating materials are given: 



