394 WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. [April 21, 



said so far is that the phenomena of sudden increase of current above 

 a certain value of electric intensity as observed by Townsend, and 

 that of corona formation, are probably due to the same causes. But 

 there are several other independent methods of determining the 

 energy required to ionize a gas. The values are commonly ex- 

 pressed in terms of the potential difference in volts through which 

 the electron must pass in order to acquire energy sufficient to pro- 

 duce an ion by collision. The value pertaining to the method de- 

 scribed above is from 10 to 12 volts. Rutherford, from the relation 

 between the heating eft'ect of radium and the number of ions it pro- 

 duces, gives the value 24 volts. Stark and Langevin by independent 

 methods conclude that the values are 45 and 60 volts respectively. 

 While the extreme values dift"er by the factor 5 or 6 it must be re- 

 membered that the actual amount of energy required to produce an 

 ion is about 5 X lO"^^ ergs, so that all of these values indicate the 

 same order of magnitude ; therefore when taken together they con- 

 stitute a very strong reason for supposing the value 5 X lO"^^ ergs 

 is close to the correct one. If this be true it is good evidence that the 

 formation of the corona is actually due to the liberation of ions from 

 the neutral molecules of the gas, when the latter suffer collision from 

 a free electron moving under the force of the electric field. That 

 the electron and not a gaseous ion or aggregate is the active agent 

 is shown by the shorter free paths of these latter which by the 

 relation already given results in a lower value of kinetic energy at 

 the time of collision than those given above. 



The writer has shown by stroboscopic methods that above the 

 critical voltage the corona begins and ends at a point on the alter- 

 nating current wave which corresponds very closely in every case 

 with this critical value. It is well known that since secondary ioni- 

 zation depends only on the velocity of the ions and thus on the 

 electric intensity, it should within wide limits be independent of 

 the number of ions already existing in the gas. The corona stops 

 sharply on the descending side of the voltage wave showing that 

 the copious ionization present during the existence of corona does 

 not aid it in persisting to a lower voltage than that at which it 

 starts. The presence of a greater or less amount of free or spon- 

 taneous ionization in the atmosphere has been advanced by some 



