1898.] Fall of Potential at the Electrodes, etc. 



19 



In these investigations, however, no experimental points were 

 obtained on those parts of the gradient curves which correspond 

 to these rapid changes at the plates, so that it is not possible to 

 say whether the fall exists in the gas near to the electrodes or 

 whether it is due to the formation of some kind of a double layer 

 on the plates themselves. 



If in passing from the gas to the electrode the potential 

 changes in a continuous manner, then evidently its variations can 

 be explained by the presence of free charges in the gas ; but if on 

 the other hand there is a sudden change of potential at the 

 electrode it is necessary to assume the presence, at the surface, of 

 a charged layer opposite in sign to that of the metal itself. 



In the previous work the nearest point at which the potential 

 was determined by the writer was one millimetre from the plate, 

 while the nearest observed by Child was five millimetres distant. 



And since it has been shown* that the free charges in the gas 

 increase rapidly as the electrode is approached, it was thought 

 possible that a determination of the potential at points nearer to 

 the surface than had been done might throw some light upon the 

 problem just mentioned. 



For this purpose a form of apparatus was constructed which 

 possesses some important improvements and permits of a deter- 

 mination of the potential at points within -^ millimetre from the 

 electrode. 



Fig. 2. 



The apparatus is shown in elevation in fig. 2, as it was set up 

 on the cover, XX', of a lead box which contained the induction 

 coil and Crookes tube. 



The electrodes between which the potential gradient was in- 

 vestigated were made of brass plates, 7"3 centimetres square, 

 which had been carefully worked flat. 



J. Zeleny, Phil. Mag. July 1898, p. 138. 



2—2 



