Cathode Fall in Helium and Argon 



II 



EXPERIMENTAL RESULTS 



Helium. — Table I gives the observed potentials at different dis- 

 tances from the axis of the cathode and the corresponding values 

 of the potential computed from the theoretical formula (8), for 

 helium at a pressure of 2.55 mm. for four different current densi- 

 ties. These are shown graphically in Fig. 4. The values of r^ 

 and Vq, the value of V at the bottom of each column, were deter- 

 mined as described above. In the first column is given the dis- 

 tance of the equipotential surface from the axis of the cathode. 

 The observed values of the potential are in reasonably good agree- 

 ment with the computed and none of them could be called dis- 

 tinctly unsatisfactory except the one for the point nearest to the 

 cathode for the greatest current density. Since nearly all the 

 measurements at that position have shown a considerable error, it 

 seems probable either that the errors due to the orientation of the 

 probe make these readings unreliable or that the setting for that 

 position is slightly within the polarization region. Confidence in 



Table I 



Aluminium Wire Cathode 



Diam., 3.2 mm. ; exposed length, 3.2 cm. ; Helium pressure zn 2.55 mm. 



the probable truth of the latter suggestion is strengthened when it 



