( 'athode Fall from Hot Carbon. 203 



fig. 4) corresponding with normal cathode-tall, and in the 

 case of low temperatures even before it. The complete 

 current-P.D. curve tor a gas at low pressure would thus be 

 represented diagrammatically in tig. 5, where the abscissa? 

 have a much greater proportional value for large currents 

 than can be shown on the same figure without crowding the 



initial stages out of reco^ 

 stage before all the 



OA represents the first 



__>re all the ions are used in carrying the current, as 



they are in AB. From B to C new ions are produced, and 



Fig-. 5. 



CU ft RENT 



when their number reaches a certain value (C) the luminous 

 discharge sets in and continues with constant cathode-fall 

 along <JD. As soon as the cathode is covered (D) the cathode- 

 fall begins to rise again until (E) the current attains a second 

 stage of saturation EF followed by a further increase of con- 

 ductivity FG. At high temperatures ; the first saturation 

 stage (AB) is realized at a much smaller P.O. owing to the 

 discharge of corpuscles from the cathode. The details of the 

 early part of the diagram are accompanied by too small 

 currents to be measurable by a galvanometer, and they have 

 been completely studied by Townsend and others. They 

 constitute the Hrst"dark discharge;" and we have now a 

 second dark discharge with very large cm-rents when the 

 positive column, and finally also the negative glow (only 

 actually observed at high temperatures), is driven back upon 

 the anode. The common characteristic of these two dark 



