394 



ALTERNATING CURRENTS 



of 100,000 volts, this bluish discharge may have formed on the 

 rod up to a point where the diameter of the rod is about % in. 

 Meanwhile the discharge from near the pointed end, and the 

 accompanying hissing sound, will have become quite vigorous. 



" L ^_ f - Ground 

 FIG. 357. Formation of corona on a tapered conductor. 



This bluish discharge is called corona. It occurs when the 

 electrostatic stress in the air exceeds about 75,000 volts maximum 

 per inch, or 53,000 volts effective per inch. At 'this voltage 

 gradient the number of electrostatic lines per unit area becomes 

 too great for the air to withstand. (See Vol. I, Chap. IX, p. 

 200.) This is the reason why corona first appears at the sharp 



point. The electrostatic flux 

 lines are more concentrated 

 at points. This is illus- 

 trated in Fig. 358, which 

 shows a conducting body 

 suspended in air, the po- 

 tential of the body being 

 considerably above ground 

 potential. The electrostatic 

 lines leaving this body are 

 indicated. They are much more dense at those parts of the 

 surface having a smaller radius of curvature. 



When air ts so highly ionized that corona forms, its dielectric 

 strength is practically nil, and the air may be considered as 



FIG. 358. Effect of radius of curvature 

 on distribution of electrostatic lines. 



