Disappearance of Gas in the Electric Discharge. 591 



the current becomes saturated, and reaches a value (i ) -deter- 

 mined wholly by the thermionic omission and not by "V. 

 The relation between i and V in this condition is shown by 

 curve I. in fig. 2, where i f is plotted against V. The curves 



Fig. 2. 



Relation between current (i) & 

 potential (v) at different pressures 



in this figure are not drawn accurately, but merely represent 

 qualitatively the charges that occur. 



If a little gas is present, curve II. is obtained. It leaves 

 curve I. at the ionization potential of the gas (Y ), and attains 

 the saturation current at a value of V less than before. But 

 the saturation current reached is the same, because the 

 current conveyed by the ions formed from the gas is an 

 inappreciable fraction of that conveyed by the thermionic 

 electrons. No luminosity appears in the discharge-tube 

 before the saturation currrentis attained : if it appears at all, 

 it is at potentials such as are used with a hard X-ray tube, 

 which are much higher than the highest used in these experi- 

 ments (600 volts). The pressure at this stage will be less 

 than *001 mm. 



If the pressure is raised to (say) *002 mm., a new feature 

 appears (curve III.). Above the ionization potential, the 

 current increases still more rapidly with the potential, but 

 before the saturation current is attained, a potential is 



