218 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



pare Fig. 154 with Fig. 152). The result is that the rupture 

 divides into many branches which penetrate into the surrounding 

 air in the form of a tree or brush. This type of discharge is called 

 the brush discharge, and it is most readily formed in a region 

 where the lines of electric force are widely divergent, as near a 

 pointed projection on a charged conductor. The brush discharge 

 forms more readily on a positively charged conductor than on a 

 negatively charged conductor, and the positive brush is very dif- 

 ferent in character from the negative brush. 



124. Electric discharge from metallic points. A body of metal 

 which has a sharp point can scarcely be charged at all, because 

 of the fact that a very slight charge on the body produces a very 

 intense electric field in the neighborhood of the sharp point, the 

 lines of force in this region break down, and the lines of force be- 

 come detached from the conductor, ending upon charged portions 

 of the surrounding air. Thus, Fig. 155^ represents a metal ball 



Fig. 155. 



with a sharp metal point, and Fig. i$$& represents the state of 

 affairs after the air has broken down in the neighborhood of the 

 sharp point where the electric field is very intense. 



The tension of the lines of force cd in Fig. 155^ pulls the 

 positively charged air away from the point, forming a blast of 

 air. If the ball is connected to an electric machine so as to be 

 continually supplied with charge, new lines of force continually 

 replace those that are broken down and a continuous blast of air 

 is produced which is sometimes strong enough to blow out a 

 candle. 



