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



with an extraneous ion. It is surprising however that this effect 

 should be noticeable at frequencies so low as 60 to 90 cycles, for 

 tliey are incomparably slower than those suggested by theory for 

 tlie vibrations within the atom. The close relation between the 

 first appearance of corona and the peak or maximum of the voltage 

 wave is natural in the light of theory, for at atmospheric pressure 

 the mean free path of an electron is about 6 X lO"^ cm. long, and 

 under a field sufficiently strong to ionize this path is traversed in 

 about 2 X lO"^- seconds. 



Perhaps the most interesting problem in connection with the 

 phenomenon of corona formation is the explanation of the greater 

 values of electric intensity required to start corona around smaller 

 wires, i. e., the upward trend of the curve of Fig. 2. Why should the 

 properties of the air change with a slight alteration in the size of a 

 conductor whose diameter is fifty thousand times as great as the 

 mean free path of a molecule? No tenable explanation has been 

 ofifered. The attraction to the conductor of oppositely charged 

 ions which pile up as it were and reduce the actual gradient below 

 that calculated, and at the same time increase the gas pressure, has 

 been suggested. Both suppositions immediately include an in- 

 fluence on corona voltage of the amount of ionization already 

 present, and this as already noticed is contrary to observation. 

 Simple calculation also will show that the charge sufficient to mate- 

 rially reduce the gradient at the surface of a conductor at corona 

 potential would require a number of ions far in excess of the num- 

 bers commonly present in the atmosphere. The writer by a sensi- 

 tive optical method could find no indication of an increase of pres- 

 sure at the surface of the conductor. It appears probable that the 

 explanation will be found in the decreasing surface of the smaller 

 conductors. Secondary ionization probably begins with the col- 

 lisions of a few electrons which have free paths longer than the 

 average. With decreasing area of conductor, the number of neigh- 

 boring electrons whose free paths exceed a certain length, and at 

 the same time are subject to the maximum electric intensity, will 

 be decreased, and consequently the corona forming electric intensity 

 must be higher. 



Johns Hopkins University, 

 April 20, 1911. 



