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THE HIGH VOLTAGE CORONA IN AIR. 



By J. B. WHITEHEAD, Ph.D. 



(Read April 24, igso.) 



Atmospheric air is an extremely good electric insulator. It has 

 low specific inductive capacity, very low conductivity, and a rela- 

 tively high electric strength, or ability to withstand breakdown or 

 spark-over between high voltage terminals. The name " corona " 

 has been given to the continuous partial breakdown of air sub- 

 jected to electric strain, and it always appears as a glow or brush 

 discharge confined to one or both high voltage terminals with a 

 region of unbroken air in between. 



When voltage is applied to a pair of parallel plates in air and 

 slowly raised, the air withstands the strain up to a definite value of 

 voltage and then breaks down completely with a heavy sparkover 

 or arc between the plates. (See Fig. i.) In this case the electric 

 field intensity or the number of volts per centimeter is uniform 

 throughout the region between the plates, being equal to the voltage 

 applied divided by the distance between them. The electric in- 

 tensity at which breakdown occurs in the air at normal atmospheric 

 pressure is about 32 kilovolts per centimeter. If needle points are 

 used, or a hollow cylinder and a wire on its axis, instead of the 

 parallel plates, a quite different behavior of the air appears. On 

 raising the voltage the air breaks down in the form of a brush or 

 glow discharge immediately around the needle points and at the 

 surface of the central wire, but the breakdown is limited to a small 

 distance and there is no sparkover or complete rupture until a 

 much higher value of voltage is reached. 



The interest in the cases of the points and the central rod and 

 cylinder lies in the fact that the electric field or voltage gradient is 

 not uniform over the distance between terminals, being highest at 



245 



PROC. AMER. PHIL. SOC, VOL. LIX, P, AUGUST 23, I92O. 



