WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 



249 



No explanation has been offered for the foregoing law, nor does 

 an obvious explanatioin suggest itself. It is not difficult, however, 

 to see that the loss should increase sharply above the critical voltage. 

 Corona first begins when the maximum of the alternating voltage 

 reaches a definite value, which, in the case of a smooth round wire, 

 is very sharply marked. Above this value corona starts on each 

 half wave at this same definite value, whatever the maximum value 

 of the voltage wave may be, and on the descending half wave 

 corona ceases at about the same value or very little below it. 

 Corona is thus periodic and with increasing voltage occupies a 

 larger and larger area at the top of successive half waves of alter- 

 nating voltage. (See Fig. 3.) In the neighborhood of the critical 



Fig. 3. 



corona forming value, when corona is limited to a short interval at 

 the crest of the wave, the loss will evidently be proportional to the 

 frequency since the number of breakdowns per second increase with 

 the frequency. However, with increasing voltage the aggregate 

 time during which the corona actually exists becomes more and 

 more nearly equal for all frequencies. As to the influence of 

 voltage, with increasing voltage the volume of corona increases, 

 and since this means increased ionization and conductivity of the 



