TRANSMISSION OF POWER BY ALTERNA TING CURRENT 395 



broken down or disrupted electrically. Under these conditions, 

 the air becomes a partial conductor and is practically valueless 

 as an insulator. 



Corona is always accompanied by the production of ozone, the 

 odor of which is readily detected. In the presence of moisture, 

 nitrous acid forms when corona occurs. The acid and ozone may 

 attack metals and other substances, such as insulating materials. 

 When corona occurs, the resulting ozone is very active chemically. 



Corona is accompanied by a dissipation of energy. If a trans- 

 mission line be operated at a sufficiently high voltage, corona loss 

 occurs. Where a line is long, the 

 loss becomes serious and must be 

 considered when the line is de- 

 signed. The loss may be reduced 

 by increasing the diameter of the 

 conductors and thus increasing 

 their radius of curvature. This 

 fa-t favors aluminum for trans- 

 mission line conductors, other 

 factors being equal. Figure 359 



, , . i i Fio. 359. Illumination of trans- 



shows the conductors of a high- mission line by corona 



voltage line illuminated by the 



corona discharge. (For a more complete discussion see "The 

 Law of Corona and the Dielectric Strength of Air," by F. \V. 

 Peek, Trans. A. I. E. I-., Vol. XXX (1911), p. 1889.) 



LIGHTNING ARRESTERS 



167. Multigap Arresters. -Abnormal vnlt.i. occur in 



power systems due to lightning discharge. >witching, short- 

 circuits, and other dist urbances. These voltage rises may dam- 

 age the system and any connected apparatus. by puncturing 

 insulation, by producing insulator Hash-overs, arcing grounds, 

 etc. It is therefore highly desirable to relieve the line of such 

 disturbances whenever possible. This is done by means of 

 lightning arresters, whose function is to relieve any abnormal 

 ise by passing current to ground and therefore prevent- 

 ing dan; he System and connected apparat 



Lightni- d between the line to be pro- 



tected and t he ground. They nin^t have four propeiti.-. They 



