THE HIGH VOLTAGE CORONA IN AIR. 



; . By J. B. WHITEHEAD. 



(Read April si, 1911.) 



The term "corona" as employed by electrical engineers refers to 

 the luminous envelope which surrounds a bare electrical conductor 

 when its potential is raised above a certain value. As the voltage of 

 long-distance transmission lines has been raised to higher and higher 

 values in order to reduce the size and cost of the conductors and so 

 increase the distance of economical transmission, a limiting condition 

 has been found in the insulating properties of the atmosphere. For 

 each definite space separation and size of conductors, above a certain 

 value of voltage the regions immediately surrounding the conductors 

 become luminous, and a power loss sets in which increases rapidly 

 with further increase in voltage. 



These facts were first noted by electrical engineers in this country 

 in 1896. It was promptly recognized that the region in the imme- 

 diate neighborhood of the conductors is subject to the greatest electric 

 intensity and that the phenomena are due to local though restricted 

 break-down of the air. This was corroborated not only by the pres- 

 ence of the luminous envelope immediately around the conductors, 

 for voltages above that at which the loss begins, but by study of the 

 eft'ect of changing the size and separation of conductors ; decreasing 

 separation and size both increase the surface electric intensity and 

 therefore lower the voltage at which loss begins. The electric inten- 

 sity at the surface of the conductor may be readily calculated in most 

 cases that occur from the voltage and from the separation and sizes 

 of the conductors. It is directly proportional to the voltage in all 

 cases. The term "corona" was first used by Steinmetz in 1898 to 

 describe the luminous enveloi)e and has been generally adopted by 

 engineers. 



^fany measurements have been made on electric power transmis- 



374 



