800 Mr. G. W. Todd on the Mobility of Positive Ions 



be possible that the positively charged particle emitted by 

 the heated phosphate required a finite time for the ionic 

 cluster to form round it. In that case, as soon as this finite 

 time became comparable with the time taken by the ion to 

 travel between the plates, the mobility would show an 

 abnormal increase. Typical results with air at pressures 

 above and below that at which the mobilities begin their 

 abnormal increase, are given in the following tables. The 

 mobility did not depend on the rate of alternation of the 

 field. 



Pressure = 1*585 mm. 



Kate of alternation 



Critical potential 

 (En). 



Mobility. 



25 

 38 

 41 

 86 

 115 

 169 



3-88 

 5-37 

 601 

 136 

 175 

 25-4 



644 

 708 

 681 

 630 

 658 

 665 



Pressure = *742 mm. 



n. 



Bo- 



u. 



96 



3-82 



2520 



111 



4 82 



2280 



149 



635 



2350 



180 



8-16 



2210 



204 



8-20 



2490 



The results of the experiments point to some change in 

 the structure of the positive ion at low pressures. It may be 

 (a) that the ion is a cluster of molecules which eventually 

 breaks up into a single charged molecule, or (b) that there is 

 a positive unit smaller than the molecule which at low 

 pressures is able to migrate from one cluster of molecules to 

 another. It is interesting with regard to (b) that Lusby * 



* Camb. Phil. Soc. xvi. p. 26. 



