598 E. M. Wellisck — Mobilities of Ions in Air. 



necessity a larger field the velocity appeared to increase faster 

 than the Held. 



The proportion of electrons increases as the pressure is 

 diminished ; but in this respect again, the field appears to be 

 without effective influence. The statement that the carriers 



are practically all electrons when — exceeds *2 is readily seen 



to be erroneous from a casual inspection of the curves, e. g., in 



curve B (fig. 7) we have p = -87 X = ™ = 30 ; - = 34'5 and 



at the same time a large percentage of ions. 



The fact that electrons can travel considerable distances in 

 dry air without attaching themselves to neutral molecules is 

 remarkable and appears to indicate that the distribution of 

 carriers between ions and electrons must be determined 

 immediately after the act of ionization. We may look upon 

 the act of ionization as resulting in the expulsion of an electron 

 with a certain velocity : this velocity decreases as the electron 

 recedes from the parent molecule owing to the positive charge 

 acquired by the latter. If in addition we regard attachment 

 between an electron and an uncharged molecule as possible 

 only when their relative velocity at impact is above a critical 

 value, it is easy to see that at low pressures when the free path of 

 the electron is relatively large the number of such attachments 

 (i. e. the proportion of negative ions) would be smaller than at 

 the higher jDressures.* Expressed in general terms, there 

 appear to be definite forces of repulsion at small distances 

 between an electron and an uncharged molecule ; when 

 however, the electron has sufficient velocity to penetrate into 

 the sphere of action of the molecule the forces are now no 

 longer repulsive but attractive. The phenomenon is in many 

 respects similar to the impact of a bullet upon a target. 



Moreover, the fact that electrons can traverse several cms. 

 of a gas at pressures as high as 10 cms without attaching 

 themselves to neutral molecules would seem to imply a fortiori 

 that the system (i. e. ion) resulting from the initial attachment 

 of an electron and a molecule would in a similar manner retain 

 its identity and pass through the gas unencumbered. Such a 

 deduction might, however, be open to objection, but what can 

 be stated with the highest degree of certainty is that both the 

 positive and the negative ions in dry air remain unchanged 

 through a variation in pressure from 1 atmosphere down to 



* In accordance with this view the applied field will begin to affect the 

 distribution when the energy acquired by the electron between collisions is 

 sufficient to cause attachment to a neutral molecule. With the moderate 

 field strengths used in these experiments such an effect would be likely to 

 occur only at the lowest pressures. 



