[mclennan-evans] 



The values of 

 Table II. 



MOBILITIES OF IONS 



25 



1-1 



V2 



have been calculated, and are given in 



Using the mean value of 



for the positive and negative 



ions respectively, from Table II, it follows that 



ki (positive) =2-52 x 10"^ cm. /sec, per volt/cm. 



k2 (negative) =4-26 x IQT^ cm. /sec. per volt/cm. 



The values of the mobilities of positive and negative ions in 

 helium at atmospheric pressure have been given by Franck and Pohl ^ 

 as 5 -09 cm. /sec. per volt/cm. and 6-31 cm. /sec. per volt/cm. respective- 

 ly. Assuming the mobilities of both types of ions to vary inversely 

 as the pressure of the gas, these determinations lead us to the values 



ki=6-28 X 10-2 cm. /sec. per volt/cm. 



k2=7 -80 X IQ-^cm./sec. per volt/cm. 

 for the mobilities of the respective ions in helium at 81 atmospheres. 

 These calculated values, it will be seen, are somewhat higher than 

 those obtained in our experiments, which would lead one to conclude 

 that the inverse pressure law was not applicable at high pressures. 



Table II 



It should be noted that, at the pressure used, the ionisation layer 

 produced by the alpha rays extended out from the polonium-coated 

 surface about 2 mm. A rigid application of the mobility equation 

 used, requires the ionisation by the alpha rays to be confined to a very 



i Franck and Pohl. Ber. d. Deut. Phys. Ges. pp. 69 and 194 (1907). 

 Sec. III. Sig. 3 



