64 J. A. POLLOCK. 



In order that the form of the curve giving the relation 

 between the current and the potential difference may be 

 realised, I give in figure 1 the plot of observations, taken 

 on an occasion when the measures were accordant over a 

 fairly wide range of voltage, which shows the character- 

 istics of the results obtained. 



In the curves there are two critical voltages; the lower 

 one may be interpreted in terms of an apparently new ion 

 with a mobility of the order of 0*01 in practical units, all 

 reference to which is reserved for a later paper; the upper 

 one, giving the minimum potential for which the current 

 has its constant value, represents the voltage when all the 

 Langevin large ions entering the pipe are just caught by 

 the inner rod. If V represents this critical potential, the 

 mobility is calculated by the well known formula given by 

 Zeleny (loc. cit.), , _ log e b/a Q 



where b/a is the ratio of the diameters of the tube and of 

 the inner rod, Q the magnitude of the air stream in cubic 

 centimetres per second, and X the length of the inner 

 electrode. 



In Table I is shown the evidence on which depends the 

 form of the curve just given. For a single determination 

 of the ionisation current the order of the observation is as 

 follows: — (1), reading of the gas meter; (2), repeated 

 measurements of the reciprocal of the ionisation current 

 in seconds per centimetre of scale reading; (3), determin- 

 ation of the leakage current with zero air stream. In the 

 great majority of cases the leak immediately after the air 

 stream was cut off has been zero; where a leak has been 

 observed the measures have been corrected for it. In the 

 table the positive sign attached to the observation means 

 that the leak was in the same direction as that of the 

 ionisation current. 



