592 Research Staff of the G. E. 0., London, on the 



reached at which there is a very sudden increase in the 

 current. The change in the current at this point is discon- 

 tinuous ; i increases from a value which is a small fraction 

 of the saturation current immediately to the saturation 

 current. At the same time, luminosity appears in the 

 discharge-tube. The potential at which this discontinuous 

 change occurs, accompanied by the development of the 

 glow, will bo termed the glow potential, Yg. Such discon- 

 tinuous changes are obtained when the pressure is still so 

 low that the ionization is an inappreciable fraction of the 

 thermionic current and the saturation current is practically 

 the same as in the highest vacuum. 



It has been said that the change is discontinuous. The best 

 proof of this statement is that, on reversing the changes of 

 potential, the changes of current are not reversed. If, 

 after the potential has been raised above V^ and the glow 

 started, the potential is reduced, the glow does not cease 

 immediately, nor does the current fall below saturation. The 

 dotted curve III/ is followed. When V has been reduced to 

 a lower value Yg', th;3 discontinuous change is reversed, and 

 the current falls once more sharply to the value which it had 

 at the same value of V when the potentials were increased. 

 When it is necessary to make a distinction in words between 

 Yg and Yg', they will be called the rising and falling glow 

 potentials. 



If the pressure is increased once more to *05 mm., curve 



IV. is obtained. It now appears at first sight as if the 



current rose continuously with the potential, until a value 



greater than the vacuum saturation current was obtained ; it 



then continues to rise yet further, but more slowly. The 



rise of the current above the saturation value in a vacuum 



is, of course, due to the occurrence of ionization producing a 



number of electrons comparable with those emitted from the 



filament. At some point before the final slower rise sets in, 



a glow appears in the lamp, which is usually fainter and 



much more difficult to see than that characteristic of curve III. 



It was thought at first that this glow entered without any 



discontinuity in the curve, but later observations indicate that 



here also there is a point of definite discontinuity, and that it 



is at this point that the glow enters. In any case, the glow 



appears at that part of the curve where the rate of increase 



of i with Y is greatest; the potential at which it appears can 



be determined with somewhat less accuracy than in curve III.; 



it will be denoted again by Yg. When there is no appearance 



of discontinuity in the curve, there is also no difference 



between Yg and Yg ; if the potential is reduced, the curve 



