THE MOBILITY OF LARGE IONS IN THE AIR. 



63 



trorneter. In some cases a leak, with zero air stream, lias 

 been observed in the opposite direction to that of the ioni- 

 sation current; this is due to a falling cell voltage, the 

 experimental arrangement being a very sensitive one for 

 detecting changes in the battery potential. By not using 

 the cells too soon after they have been charged the leak 

 may generally be avoided. 



The calculation of the mobility involves a determination, 

 for the air stream used, of the minimum potential difference 

 between the tube and its inner rod for which the ionisation 

 current has a constant value. This critical potential is 

 found from the plot of observations of the currents with 

 increasing values of the potential difference, but, as the 

 number of ions naturally in the air is usually very variable, 

 it is only on occasions that the observations are accordant 

 enough for the purpose of the investigation. 



Figure 1, 









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