L. G. Raub 



agreement and do not show the marked variations with change of 

 current density that was found with heHum. However, the actual 

 vahies of mobihty are found to be about five times as large as 

 the value/*^ 1.04X10^, obtained by computation from Franck's 

 measurements made at atmospheric pressure. 



Table IV 

 Aluminium Wire Cathode 

 Diam., 3.2 mm. ; exposed length, 3.2 cm. ; Argon, pressure = .75 mm. 



The values shown in Table IV and Fig. 7 were taken with the 

 same sample of gas as the readings of Table III, but after it had 

 been standing in the apparatus several days. The computed and 

 observed potentials are in good agreement, but markedly lower 

 than the values found in Table III. This was not surprising, since 

 it is a well-recognized fact that a slight change in the cathode 

 surface may produce a marked change in the total potential fall, 

 but this does not involve any change in the potential gradient in 

 the Crooke's dark space, nor would it have any influence on the 

 values of the mobility in that region. A comparison of Figs. 6 

 and 7 shows that the curves of Fig. 7 are flatter than those in Fig. 6, 

 the drop of potential through the Crooke's dark space being 

 ^''J. Franck, Bcr. d. D. Phys. Gcs., 8, p. 291, 1910. 



