250 WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 



air, the current also increases. The power loss being the product 

 of voltage and the current increasing with the excess of voltage 

 above a certain critical value, we should therefore expect the loss 

 to increase as some power of the voltage higher than the first but 

 that it should be as the second power of the excess voltage in not 

 evident. 



Further studies of the nature of the power loss and of ^^the 

 exact law governing it are greatly needed. It seems probable that 

 the observations which have already been made on transmission lines 

 are about as satisfactory as can be obtained in this way. Since it is 

 important to adopt a voltage well below that at which corona forms, 

 and since the values at which corona will start on clean wires are 

 now accurately known, it does not appear likely that transmission 

 engineers will go very far in further investigation. There is here, 

 however, again an admirable opportunity for laboratory study. 



It will be seen therefore that it is important to know accurately 

 the laws connecting the physical constants of an electric circuit and 

 the voltage at which corona begins. A number of experimental 

 studies of this question have been made and some of the earlier of 

 these were reported to this society by the present writer several 

 years ago. Since then the law of corona formation has been well 

 established and it is the purpose of this paper to give the results 

 of some of the more accurate measurements. These measurements 

 have been made with the assistance of an instrument in which the 

 otherwise troublesome corona pheonmena have been turned to use- 

 ful account as a means for the accurate measurement of high alter- 

 nating voltage. The accurate measurements referred to were made 

 on this instrument, which is called the " corona voltmeter." 



The Corona Voltmeter. — The corona voltmeter makes use of the 

 fact that corona forms on a clean round wire in air at a sharply 

 marked definite value of voltage, dependent in a simple relation on 

 the density of the air. The range of the instrument is extended 

 to wide limits by enclosing the wire and cylinder and varying the 

 density of the air. 



The essential elements of the instrument are a central rod, or 

 wire, on which corona forms, another concentric cylinder forming 



