356 



PHENOMENA, ATOMS, AND MOLECULES 



^obs = 0.04 is made up of patches whose concentration is greater than that 

 corresponding to ^ = 0.04 and areas of nearly bare tungsten. 



Although the above types of experiments may give us a more detailed 

 picture of the tungsten surface, it must be concluded that they are not 

 suited for studies of Vp from the main homogeneous part of the tungsten 

 surface. As already shown, experiments made under steady conditions 

 (vp -|- Vffi = \ia) do give values of Vp in accord with theory. 



The films obtained in the manner described above by evaporation of 

 ions in accelerating fields illustrate the existence under these conditions of 

 films which do not conform to the surface phase postulate.^'^ 



Killian ^^ has obtained current-voltage characteristics for potassium and 

 rubidium ions in the region of space charge limitation. He showed that the 

 theoretical equation was followed and also gave curves exhibiting a re- 

 markably sharp break at the saturation voltage. Figs. 20 and 21 were ob- 

 tained for caesium ions and electrons in the present investigation. To avoid 

 question as to the correct zero of potential the 2/3 power of the current has 

 been plotted against the voltage. The break at saturation for ions is so 

 sharp that it can be almost entirely accounted for by the small voltage drop 

 along the central part of the filament. For electrons the transition occurs 

 much more gradually and must have another explanation. With the masses 

 of the electron and of the single Cs atom, the theoretical slopes were 



Oj 



Op ^'T) 



%/^^ 

 ^ 



Gi 



I' 



Fig. 20. Positive ion currents limited by space charge for Cs ions. 



calculated. For electrons the calculated slope was 1.14X16"^ and the 

 observed 1.05 X 10"^. For ions the calculated slope was 1.82 X 10"^ and 

 the observed value was 1.65 X io~^. The agreement is satisfactory since 



^"^ A more detailed analysis of this postulate and its implications in connection 

 with a phase rule for adsorption has been presented by I. Langmuir, in reference 16. 



