258 WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 



and the other for values of l/^/hr above 2.26, the equation in this 

 case being 



8-56 

 £ = 32.96+-^. (7) 



The sharp change in the slope of the linear relation between E/^ 

 and i/V^'' finds its explanation in the fact noted above as to the 

 difference of behavior as regards corona formation between positive 

 and negative corona forming wires, or rods. As has already been 

 stated, the form of the law is the same for both positive and nega- 

 tive rods, but the constants of formula (2) are different. This is 

 equivalent to saying that for values of i/\/hr below 2.26 negative 

 corona appears first and the law is as given by formula (6). For 

 values of i/-\/hr above 2.26 positive corona appears first and the law 

 is as given by formula (7). 



All of the measurements leading to these results were made in 

 terms of laboratory standards and by use of the best available 

 equipment and experimental methods. No account is given here 

 of the various experimental difficulties, precautions and calibrations, 

 a complete account of these being given in a forthcoming paper in 

 the Journal of the American Institute of Electrical Engineers, May, 

 1920. The conditions of accuracy are discussed there and show 

 that the foregoing formulae are probably accurate within an ex- 

 perimental error of considerably better than y^ per cent. 



Considered as an instrument for measurement of voltage, it will 

 be noted from Table i how definitely the readings repeat them- 

 selves. The law of corona, having been determined by the accurate 

 methods outlined above, the corona voltmeter may therefore be 

 used itself as an instrument for the direct measurement of voltage, 

 in fact its calibration is inherent in its dimensions and it becomes 

 a natural secondary standard. 



Fig. 8 shows the exterior of a corona voltmeter suitable for 

 measurements of voltage up to 200,000 volts. The accurate meas- 

 urements referred to above were made with this instrument. It is 

 9 ft, 10 in. high, of which 3 ft. is in the insulating terminal. The 

 outside diameter is i ft. 10 in. An instrument suitable for voltages 



