Potentials of Gases observed in Thermionic Valves, 415 



filament there was a spiral molybdenum wire of diameter 

 0*4 mm. coiled into a helix of diameter 4*5 mm., having 

 four or five convolutions per cm. length. Outside this 

 was a nickel cylinder 10 mm. in diameter. In such a 

 tube it' the vacuum is high enough for the effects of ionization 

 to bQ inappreciable, it is necessary to apply a potential of 

 some 60 volts to the combined collecting electrodes in order 

 to reach the saturation value of the space-current when the 

 cathode is, as in these experiments, hot enough to give a 

 saturation current of 30 milliamperes. The lower parts of 

 the current-voltage curve, well away from this saturation 

 value, are observed to have a form sensibly independent of 

 the cathode temperature, but showing a slight bodily move- 

 ment, not accompanied by change of form, in the negative 

 direction parallel to the potential axis when the temperature 

 is increased. 



This movement is ascribable in some measure to increase 

 in the average velocity of projection of the electrons consti- 

 tuting the current, but in large part also to increase of the 

 length of the active part of the filament owing to a closer 

 approach to the negative end of the filament by the point 

 where the filament surface, in spite of the cooling effect of 

 the end supports, first becomes hot enough to emit. 



If, how T ever, gas is present in considerable quantity, a 

 point can be found, by giving to the applied potential a 



Fiff. 2 



At 



suitable value dependent on the gas used, where there is 

 a marked departure from the form of the high-vacuum 

 characteristic, an upward turn of the curve making its 

 appearance, as shown at A in fig. 2. The high-vacuum 



