Nolan and Harris — lonisation in Moist and Dry Air. 37 



About the relative proportions of the different ions present, it is difficult 

 to make a definite statement. The last-mentioned ion is certainly present 

 in considerable quantities ; but the faster ions are also present in quantities 

 which seem large in view of the low mobility value generally attributed to 

 ionisation in this gas. This question takes on a different aspect, however, in 

 the light of further information. 



Eocamination of Undried Air ly the Alternating Field Method. 



Our experience with CO2 showed clearly that the air-stream method was 

 not suitable, without extensive alterations in our apparatus, for dealing with 

 ionisation in gases other than air. To deal with various gases, to examine 

 dry gases, and especially to obtain by an independent method confirmation 

 of the results already obtained, it was desirable to use some such method as 



Fig. 3. 

 Biagram of apparatus used in alternating-field method of nieasuiing mobilities. 



that of Rutherford, as modified by Franck. Various forms of this method 

 were tried, and finally the very simple method described below was found 

 satisfactory. A diagram of the apparatus is given in fig. 3. It is not drawn 

 strictly to scale. 



The alternating field is applied between a sheet of perforated zinc and a 

 brass plate, distant exactly 1 cm. below it. This plate is furnished with an 

 earth-connected guard-ring, and the perforated plate is supported by three 

 ebonite pillars, which rest on the guard-ring. The ionisation is produced by 

 a thin layer of polonium on the upper surface of a small disc of bismuth, 

 which rests on the perforated plate. Ions of one sign are driven into the 

 lower space by a field applied between the perforated plate and a plate 4 cms. 



