191 1.] WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 391 



The writer has conducted a short series of tests between 6° and 

 41° C. on a .27-cm. wire for the purpose of obtaining a correction 

 factor for his various observations as taken at different tempera- 

 tures. The result as stated in the paper already referred to is that 

 the relation is linear and that for each degree rise or fall from 21° 

 C. there is a lowering or raising in the value of the critical voltage 

 of 0.22 per cent. ; Ryan's results indicate 0.27 per cent, for this 

 value. Expressed in terms of surface intensity in kilovolts per centi- 

 meter and temperature in degrees Centigrade the writer's results 

 may be expressed by the formula : 



KV./Cni. = 6i — .iT,2t. (6) 



In view of the observations of the effect of variation of pressure on 

 different sizes of wire, it is not improbable that the constants of 

 equation (6) will also vary with the size of wire. Further investi- 

 gation in this direction is therefore desirable. 



Moisture content up to amounts quite close to saturation have 

 no effect on the values of voltage at which corona begins. While 

 there is still some dissent from this opinion among electrical engi- 

 neers, the author's results on this question, described in the earlier 

 paper, appear very conclusive, and have been widely accepted. An 

 influence of moisture on the amount of power loss above the critical 

 voltage appears quite probable, in the light of the ionization theory 

 in which the mass of the ionic carriers, which make up the current 

 are an important factor in its value. 



Discussion. 



So far as the question of the value of voltage at which corona 

 will start on a given transmission line is concerned, it is probable 

 that a solution will be reached sooner or later by means of experi- 

 ments of the general character as those described above, supple- 

 mented by observations on existing lines. Also, there is good reason 

 to suppose that a comparatively simple law will be found. For the 

 surface intensity for any arrangement and size of cylindrical con- 

 ductors, corresponding to a given voltage, may be expressed in terms 

 of these constants ; and the critical or corona intensity, under stand- 



