at the Electrodes in Vacuum- Tube Discharge. 637 



Results at Cathode and Anode Compared. 



With the above observations at the anode, the complete 

 -similarity in conditions existing at the two electrodes seems 

 "to be established. From the view taken that the drop is 

 -caused by the velocity of impact of the ions, its striking 

 difference in magnitude at the two electrodes calls for a 

 .special explanation. If this difference in magnitude were 

 due simply to a different impact velocity, that of the positive 

 ion must be considerably larger than that of the negative. 

 Experiments of Zeleny* and others on ion velocities seem to 

 preclude that possibility. The}' have fonnd in general that, 

 <under the same forces, the negative ion moves with a greater 

 velocity than the positive. A more plausible explanation 

 snay be given from the view that the positive ion possesses a 

 much greater mass than the negative. It is evident that for 

 the same velocity of impact the greater the mass the greater 

 the force of elastic reaction, and in this case therefore the 

 greater the electric forces at the electrode necessary to bring 

 the ion to rest in a given time. Under such conditions the 

 drop at the cathode would be larger than at the anode, and 

 likewise, the cathode possess a greater rate of increase in 

 drop with increasing current and decreasing gas-pressure. 



Another fact pointed out by the writer (Wied. Ann. I. c.) 

 demands attention in this connexion, namely : Of two metals 

 in the same gas the one showing the greater drop of potential 

 as cathode possesses the smaller drop as anode. The drop 

 of potential at an iron anode is, under the same conditions, 

 smaller than at an aluminium. In fig. 5, as cathode the con- 

 ditions are reversed — aluminium possesses the smaller drop. 

 An explanation is suggested by the similarity existing between 

 the series of metals arranged according to the magnitude of 

 their drop as anode, and the contact-potential series. The 

 two series seem to be identical : — 



Series with 



Series with Increasing 



^creasing Drop at 



Attraction for 



Anode. 



Negative Electricity. 



Aluminium. 



Aluminium. 



Magnesium. 



— 



Zinc. 



Zinc. 



Iron and Copper f. 



Iron. 



— 



Copper. 



Platinum. 



Platinum. 



* John Zeleny, Phil. Trans, vol. cxcv. p. 160. 



f The extreme sensibility of copper in that its drop at anode increases 

 very rapidly with use, raised it to a level with iron, though the contact 

 •series places it below. 



