﻿640 Prof. J. A. Pollock 



on a 



measures advisable in the present instance. The mobilities 

 have been calculated, without correction, from the formula 



_( log,6/a)Q 

 U ~ 2ttYX ' 



where bja is the ratio of the radii of the tube and inner rod, 

 Q the air-stream in cubic centimetres per second, V the 

 critical potential difference between the electrodes corre- 

 sponding to a special value of X, the distance of an end of 

 an electrode from the mouth of the tube. 



In the curves like that in fig. 1 there are two critical 

 voltages. The upper one A, the minimum potential difference 

 for which the current has its constant value, represents the 

 voltage for which the extreme ions of a certain class just 

 reach the further end of the long electrode, counting from 

 the mouth of the tube, the extreme ions being those which 

 enter the tube at a distance b from its axis. The ions here 

 are the large ions of Langevin. 



The other critical potential B gives the voltage at which 

 the extreme ions of another class just fail to reach the near 

 end of the same electrode. 



When the short section of the inner rod, at the mouth of 

 the pipe, was attached to the electrometer, only one critical 

 potential occurs, as shown in fig. 2. The value here is the 

 voltage when the extreme ions of a certain class just reach 

 the further end of this short electrode. It was found that 

 the critical voltages in the two latter cases refer to the same 

 class of ions, the calculated value of the mobility being in- 

 dependent of the particular arrangement of apparatus which 

 was used. It is the ions of this class which form the subject 

 of this paper. 



The values of the mobilities which have been determined 

 are given in Table I., T being the temperature in centigrade 

 degrees, p the vapour-pressure, p/F the relative humidity, 

 and N the number of these ions per cubic centimetre under 

 the circumstances of the experiment. A considerable number 

 of measurements were made with artificially dried air when 

 the value of the humidity was about 33 per cent. To keep 

 the table within reasonable limits only the extreme measures 

 for these humidities have been given, but the omitted results 

 are included in figs. 3 and 4. The numbers have been re- 

 duced to standard pressure on the assumption that at constant 

 temperature the mobility varies inversely as the density of 

 the air, but this and the corresponding temperature correction, 

 calculated on whatever basis, are too small to be of the 

 slightest consequence in connexion with the present measures. 



