WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 



259 



up to 300,000 volts and above is now under construction and will 

 shortly be put into operation by a well-known hydro-electric power 

 company. 



In operation the corona voltmeter may be used in two ways. 

 First, it may be set for any desired value of voltage by adjustment 

 of pressure in the instrument. In order to do this the temperature 



Fig. 8. Corona Voltmeter for 200,000 volts. 



of the air in the instrument is read and the value of the pressure 

 for any desired value of voltage may then be computed from 

 formulae 3, 6 and 7. Ordinarily this will be done through refer- 

 ence either to a curve or a table prepared from the formula. The 

 pressure having been set at the proper value the voltage is then 

 slowly raised until corona appears. This is the more common 



