Ionizing Processes in a Point discharging in Air. 283 



The resulting values of f are given by the line marked 

 Field in Curves III. (PI. IV.), and are in good accord with 

 this theory — rather surprisingly good accord considering the 

 great difficulty of determining exactly when the glow started 

 in the case of the observations at the lower fields. (In the high 

 fields it began more suddenly and was brighter.) There is 

 an obvious halting place at a field of 250 E.S. units in the 

 falling of /o as x decreases. 



The fact of the field being thus constant over a certain 

 range of x values does not, however, prove that there is no 

 glow below this particular field. The amount of light in 

 the glow depends on the current arriving at the point and 

 on the field there ; and if the current happened to be constant 

 for this range of x, a constant field might merely mean that 

 until this field was reached the glow was too weak to be seen, 

 and not that it was absent altogether. 



To meet this objection we have plotted in Curves III. the 

 current received by P at the moment the glow became 

 visible. Starting with x = (N and P opposite one another), 

 it will be seen how very far from constant this current is. It 

 falls rapidly as x increases until the value of / begins to change. 



The subsequent rise and fall of the current curve at 

 higher values of x is attributable to the fact that, when x is 

 comparable with y, P receives most of the current on its 

 sides. For as soon as the N ions are too few to give a 

 detectable glow without a higher field f and therefore the 

 current from N increases, the result being that P receives a 

 larger total current than before, though its end of course 

 does not. At still higher values of x, f becomes constant, 

 and the same as for R without N" ; the current now falls off 

 once more as it should, and the end of P presumably receives 

 no ions at all. 



Although, as already explained, if we alter y instead of x, 

 we cannot trace the field curve up to the top, it is possible 

 to obtain the horizontal part at/—. 



In Curves IV. are plotted the results of experimenting in 

 this way with the same two points. The field curve becomes 

 horizontal at about 240, which agrees with the 250 just 

 obtained for /— , and the current curve also shows the same 

 sort of behaviour as the one in Curves III. 



But the most interesting feature of Curves IV. is the way 

 in which the field drops below the horizontal when N is 

 brought nearer to P than about a centimetre. At this point 

 the rise o£ the current, when y is decreased, becomes less 

 marked, and when N is 0*7 cm. from P the rise changes to a 



U2 



