232 Dr. L. B. Loeb on the Relative Affinity 



a constant, but that it varied according to the equation 



_ 571,000 . 



^~21 + 760V/ P d w 



Here V is the voltage, p is the pressure in mm., and d is 

 the plate distance. This result at first sight seems strange. 

 Townsend *, however, had pointed out that for electrons 

 making elastic collisions with gas molecules W, the velocity 

 of the electrons among the gas molecules, must increase as a 

 function of the field strength. If this occurs the mobility K' 

 of the electron will, according to him, decrease as the field 

 increases. These results obtained on electrons in N 2 show 

 that it is possible to ascribe the apparent variations in n to 

 real variations in K' ; for they show that K' does in one 

 instance vary as a function of the pressure and the field. It 

 is very doubtful whether the above results in N 2 ma y De 

 rigorously carried over to other gases for the purpose of 

 -evaluating n. Since, however, K' has been shown to be a 

 function of V/d, and of p, it is certain that in comparing the 

 values of n for different gases, these values of n must be 

 chosen from data for which the field strength and the pres- 

 sure are as nearly the same as possible. In determining the 

 values of n to be used in the table, care has been taken to 

 choose values obtained when the field strength is close 

 to 20 volts per cm. and the pressure is as near 75 mm. as 

 practicable. 



In the recent work on the mobility of electrons in nitrogen 

 it w T as found that the value of K' at a field strength of 

 20 volts/cm. and a pressure of 75 mm. lies close to 

 2800 cm./sec. This requires that the absolute values of n 

 heretofore determined on the basis of K' = 200 must be 

 modified. Furthermore, it is obvious that in order to apply 

 the equation to the evaluation of n in other gases, where the 

 value of K' is not known at all, some tentative value of K' 

 must be assumed. It is known that the mobility of the 

 negative ion in different gases does vary by small amounts. 

 In the evaluation of n from these experiments the value of 

 K' for the electrons in the different gases will be taken as 

 the value of the electron mobility in N 2 {e.g. 2800 cm./sec.) 

 multiplied by the ratios of the mobilities of the negative 

 ions in the gas to the mobilities of negative ions in ~N 2 



(i.e. K 7 g2LS = ~K.'- S[i) ~-~— , where K represents the ionic 



mobilities). The values of n, the constant of attachment 



* J. S. Townsend, Elect, in Gases, p. 124 (Oxford, 1914) j Phil. 

 (6) xl. p. ,505 (1920). 



