191 1.] WHITEHEAD— HIGH VOLTAGE COROXA IN AIR. 375 



sion lines in efforts to determine the law connecting the voltage at 

 which loss begins with the physical constants of the line. These 

 measurements have shown marked Inconsistencies among themselves, 

 the results on the same lines on different days being often at variance. 

 A number of laboratory investigations, in which the widely varying 

 conditions of a transmission line are under control, have naturally 

 followed. They have indicated with a rather wide variation in nu- 

 merical values that the critical voltage or voltage at which corona 

 begins on round wires varies inversely as the temperature and directly 

 as the pressure : also that the electric intensity under which the air 

 near the surface of the conductor breaks down has not a constant 

 value but increases markedly for conductors of small diameter ; and 

 further that the value of the intensity at which break-down begins is 

 that corresponding to the maximum value of the alternating wave, 

 and is independent of the material of the conductor. 



The general nature of the influence of temperature and pressure 

 could probably have been predicted from numerous investigations of 

 the discharge of electricity through gases ; the cjuantitative relations 

 for pressures near that of the atmosphere do not, however, appear 

 to have attracted the physicist, nor indeed have they as yet been 

 satisfactorily determined for the voltage of corona formation by- 

 experimental engineers. The accumulated results of physical inves- 

 tigation and theory, however, oft'er no obvious explanation of the 

 rise of the critical surface intensity for smaller wires, nor of the 

 influences of the form and frequency of alternating voltage. The 

 fact that the corona voltage is that corresponding to the maximum 

 value of the alternating wave has been proven by stroboscopic 

 methods and by the use of distorted wave shapes. It indicates that 

 the time element involved in the process of break-down of the air 

 is short compared with the periods of the common alternating cur- 

 rent circuits. The apparent sharpness of the connection removes 

 many objections to the use of the alternating electromotive force as 

 a means of investigation, and renders available its many advantages. 

 It is only necessary to know the shape of the alternating wave and 

 this may be obtained readily by several well known methods. Effec- 

 tive values as read on direct reading instruments may thus be used 



