The Discharge of Electricity through Gases. 537 



A word is necessary to explain what I mean by a considerable fall 

 of potential at the electrode. In electrolytes the fall of potential 

 within the double layer is of the order of magnitude of 1 volt ; such a 

 fall in itself would hardly make itself apparent in our measurements 

 in gas discharges. The observed fall of potential in vacuum tubes 

 at the kathode is measured in hundreds of volts, but it will appear 

 that within molecular distances the fall is many times smaller than 

 for electrolytes, so that the normal force at the kathode may be less 

 in gases than in electrolytes. The difference between an electrolyte 

 and a gas is this : that, while in a liquid the fall of potential only 

 takes place within a molecular range, in the gas it continues 

 through a measurable distance. The cause of this, no doubt, has 

 to be explained, and an attempt will be made to do so. At present it 

 is sufficient to point out that a large fall of potential within, 

 say, a centimeter of the electrode is not inconsistent with the much 

 smaller fall observed within the double layer of liquids. It must 

 also be kept in view that all measurements of potential in the 

 gas are made by means of secondary electrodes. If a double layer of 

 the same moment covers both the kathode and the secondary elec- 

 trode, it would escape detection. There are some delicate questions 

 involved in this which will be discussed in the complete paper. The 

 main conclusions drawn in the text are not altered by the fuller dis- 

 cussion of the problem. A few facts may be quoted in support of 

 the view I have taken concerning the effect of chemical action at the 

 kathode. A flame conducts, because in the flame the molecules are 

 broken up by independent chemical action ; we find, in consequence, 

 that an electrode cau freely discharge positive electricity. On the 

 other hand, a glowing piece of charcoal may act as electrode, because 

 it takes part in a chemical action, but it is the negative electricity 

 which now escapes freely. 



In a vacuum tube, whatever chemical action may take place at the 

 kathode diminishes the fall of potential, as appears from Warburg's* 

 observations, who finds that electrodes of zinc and copper in hydrogen, 

 when first introduced and therefore probably covered with a film of 

 oxide, show a considerably smaller fall of potential than after they 

 have been used some time, when the oxide film may be supposed to 

 have been removed. In a similar manner a small admixture of 

 moisture iu nitrogen tubes, causes a considerable reduction in the 

 fall of potential. As regards red-hot copper wires, we have seen 

 that the passage of negative electricity from the electrode to the gas 

 for small electromotive forces may be prevented altogether, if chemical 

 action is prevented, while the electrode may still act as anode. Here 

 it is the combined effect of temperature and electric stress that splits 

 up the molecules into ions. 



* ' Wiedemann, Annalen,' vol. 31, p. 592 (1887). 



