260 WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 



method, as the instrument is chiefly of vahie in the testing of high 

 voltage apparatus, such as transformers and insulators, in which 

 case it is desirable to apply a definite test voltage as determined by 

 the rating of the apparatus in question. 



Second^ the corona voltmeter may also be used for measuring 

 an unknown voltage by adjusting the pressure for a value of voltage 

 known to be higher than that to be measured and then gradually 

 lowering the pressure until corona appears. 



The instrument as illustrated in Fig. 8 provides a ready means 

 for removing the corona rod either for cleaning or for the sub- 

 stitution of one of a different diameter. A clean rod may be used 

 for many hundred observations without deterioration of its surface. 



The corona voltmeter offers many important advantages over 

 existing methods of measuring high voltage. The only other 

 method of direct measurement available is that of the sphere gap 

 or spark between metal spheres. This method is subject to serious 

 error due to the proximity of surrounding objects, and has a dif- 

 . ferent calibration curve for the cases of one sphere grounded and 

 both spheres insulated. It also has the serious disadvantage that it 

 necessitates a spark discharge across the circuit and that the high 

 voltage terminals must be manipulated for each new adjustment. 

 The casing of the corona voltmeter is grounded at all times, and 

 provides a complete electrostatic screening making the instrument 

 free from all types of outside disturbance. It causes no discharge 

 and draws no current from the high voltage terminals. Changes of 

 setting to meet new values of voltage are accomplished merely by 

 changing the air pressure in the instrument. A number of lesser 

 advantages need not be enumerated here. 



Laboratory of Electrical Engineering, 

 Johns Hopkins University. 



