Mobility of the Positive Ion at Low Pressures. 169 



low pressures (Joe. cit.), the writer has suggested that cor- 

 puscles have to travel a very considerable number of molecular 

 free paths from the source of formation before an equilibrium 

 state of ionization exists. If the distance over which the 

 mobility is measured is too small, and the source of forma- 

 tion of the corpuscles is at or near one boundary of this 

 distance, the proportion of ions reaching the other boundary 

 as aggregates of gas molecules will be smaller than if the 

 distance had been sufficiently large for equilibrium to have 

 been reached. Thus the mobility would appear to be higher 

 than normal. Increasing the distance between the boundaries 

 of the alternating chamber gives more time for the ions to 

 attain equilibrium conditions, hence the mobility departs less 

 from the normal value. 



Now if the positive ion is of the same nature as the 

 negative ion, there should also be an increase in its mobility 

 when the pressure is sufficiently low and the distance between 

 the plates of the measuring chamber small enough. More- 

 over, the mobility should approach nearer the normal value 

 when the boundaries are much farther apart. This has been 

 shown to be the case in the tables and curves above. The 

 fact that abnormality sets in at much lower pressures than 

 in the case of the negative ion, indicates that the charged 

 nucleus of the positive ion is of much bigger dimensions 

 than that of the negative ion. 



Some measurements were made with the pressure constant 

 and rather low, using different rates of alternation of the 

 electric field, and it was found that after the frequency had 

 passed a certain value the mobility began to increase. An 

 increase in the rate of alternation means of course an increase 

 in the electric field, if the ions are to traA'el the same distance^ 

 As the potential increased a stage would be reached when 

 the violence of collision of the ions would prevent the for- 

 mation of the normal clusters of molecules, and thus a 

 different condition of equilibrium would result in which the 

 size of the clusters on the average would be smaller than 

 usual, and the mobility at this stage would consequently 

 show an increase. Further, the proportion of ions present 

 with sufficient energy to ionize the gas molecules by collision 

 would help in the same direction. 



A set of readings showing the relation between the frequency 

 of alternation and the mobility when the pressure was con- 

 stant is given below (p. 170). The pressure for this particular 

 set of readings was *365 mm., and it will be noticed that after 

 the frequency had passed 150 per second the mobility began to 

 increase. At lower pressures the increase began at lower 



