132 ADVANCED ELECTRICITY AND MAGNETISM. 



82. Disruptive discharge. Dielectric strength. When the 

 electromotive force which charges a condenser is increased more 

 and more, the dielectric of the condenser is eventually broken 

 down; this break down occurs in the form of an electric spark, 

 it discharges the condenser, and it is called a disruptive discharge. 

 By a condenser is here meant two metal bodies of any shape 

 separated by insulating material. The electromotive force re- 

 quired to break down a dielectric depends upon three things, 

 namely, (a) the shape of the metal bodies, (b) the minimum dis- 

 tance* between the metal bodies, and (c) the nature of the dielec- 

 tric. The dependence upon the shape of the metal t bodies is 

 illustrated by the fact that a given electromotive force will 

 produce a much longer spark between points than between flat 

 metal surfaces. In the following discussion the dielectric is 

 assumed to be between flat metal plates. 



When the dielectric is perfectly homogeneous like air or oil, 

 the voltage required to break it down is very nearly proportional 

 to its thickness, and the voltage required to break down such a 

 dielectric divided by the thickness of the dielectric is called its 

 specific dielectric strength. Thus the specific dielectric strength 

 of air is about 35,000 volts per centimeter. When the dielectric 

 is non-homogeneous the voltage required to break it down is 

 not even approximately proportional to its thickness. The most 

 familiar example of a non-homogeneous dielectric is the material 

 which is used for insulating the windings of dynamos and trans- 

 formers. This material is made up of layers of cloth and 

 varnish and mica with occasional layers of air.- 



If a tank is made with one wall of porous material like unglazed 

 earthenware, the pressure of the fluid in the tank has three 

 important effects upon the wall, namely, (a) a certain amount of 

 fluid soaks through the wall, (b) the wall is slightly elastic and 

 it yields a little to the fluid pressure, and (c) the wall has a certain 

 ultimate strength and it will burst if the pressure exceeds a 



* This is not true when the distance is very small or when the bodies are in a 

 very good vacuum. 



