Cathode Fall in Helium and Argon 



13 



Using the measured values of r^ and F„, the measured potential, 

 V, of one other equipotential surface, near the cathode glow, but 

 not directly at the edge of the cathode glow, and the current I^, 

 and solving the equation for the value of U, we can obtain the 

 value of the mobility for each current density. 



Table II 



Aluminium Wire Cathode 



Diam., 2)-^ mrn. ; exposed length, Z-^ cm. ; Helium pressure = 1.55 mm. 



Dist. 



7i =.625X10-3 

 Amp. 



/i =.94X10-3 

 Amp. 



/i = i. 25X10- 

 Amp. 



/i =1.56X10-3 

 Amp. 



ir) 



Cm. 



Obs. 



Calc. 



Obs. 



Calc. 



Obs. 



Calc. 



Obs. 



Calc. 



.20. . 

 .22. . 

 .24. , 

 .25. . 

 .26. . 

 .27. . 

 .28. . 

 .29. . 

 .30. . 

 .31. ■ 

 .32. . 

 •34- • 

 .36.. 

 •375 

 .38.. 

 .386 

 •39- • 

 U... 

 Ui.. 



151-7 

 178.0 

 199.2 

 218.5 

 226.2 



244-5 

 256.2 



267.3 



273-5 

 285.0 

 292.5 



300.8 



150-3 

 176.6 

 200.3 



219-7 

 228.6 



245.8 

 260.2 



273-1 

 278.6 

 287.6 

 295-2 



179-0 

 208.5 

 233-0 

 251-5 



274.0 

 289.9 



304-7 



314.2 

 322.5 

 330.0 



179.7 

 207.2 

 232.7 

 253-4 



272.3 



288.3 



303-1 



314-I 

 323.8 

 330.2 



299-4 



300.8 



334-8 

 336.0 



335-5 

 336.0 



203.0 

 236.8 

 258-5 

 274.6 



298.3 



321.8 



322.0 



332.2 

 339-8 

 343-5 



350.5 



206.7 

 232.7 

 255-6 

 276.4 



294-3 



308.8 



321.6 



332.7 

 340.7 

 347-4 



350-5 



222.5 

 259.8 

 280.3 

 296.2 



318.4 



339-8 



355-8 



362.5 

 367-7 

 373-2 

 374-5 



226.3 

 255-2 

 279.1 

 299.2 



317-2 



333-5 



346-4 



357-6 

 366.3 

 372.2 

 374-5 



2.01 Xio3 

 3. II Xio3 



2.68 Xio3 

 4.15 Xio3 



3.84X10' 

 5.95X10* 



4.17 XiO'' 

 6.46X103 



When the mobilities are thus computed the values obtained at 

 the different current densities are not found to be in agreement 

 as might be expected from the theory, but the value of the mobility 

 obtained increases as the current density is increased. The sug- 

 gestion occurs that this may be due to the nitrogen present as an 

 impurity. Since the ionization potential of nitrogen is lower than 

 that of helium, it might be expected that at the lower currents the 



