2l6 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



breaking point of the intervening dielectric. The rupture of the 

 dielectric starts in the region of greatest electric stress,* as indi- 

 cated by the short thick line projecting from the surface of A 

 in the figure. Along the line of this rupture the dielectric is a 



Fig. 152. 



good conductor, and the lines of force on all sides move sidewise 

 into the rupture as indicated by the arrows, producing a greatly 

 intensified electric field at the end of the rupture so that the rup- 

 ture extends further and further until it reaches B. 



This extension of an electric rupture or spark through a region 

 in which the intensity of the electric field is originally much be- 

 low the breaking value of the dielectric is analogous to the fol- 

 lowing : A pane of glass is slightly bent and then scratched near 

 one edge so as to start a crack. The effect of this crack is to 

 greatly intensify the stress in the glass at the end of the crack 

 and the crack therefore quickly runs across the pane. 



When the electric rupture has extended itself across from A 

 to B in Fig. 152, a conducting line or path is established from 

 A to B t and all of the charge on A and B disappears, that 

 is to say, the electric field between A and B disappears. 



123. The brush discharge. The discharge in air from a body 

 of metal which stands at a distance from surrounding bodies is in 

 some respects different in character from the spark discharge be- 

 tween two oppositely charged conductors which are not too far 

 apart. Figure 153 represents the lines of force spreading out 



* This rupture always starts in air at the surface of the positively charged ball, un- 

 less the surface of the other ball Is much more sharply curved. 



