Gases on Contact Potential Difference between Metals. 173 



These figures suggest that if the effect is real it is in the 

 opposite direction to that observed with platinum, i.e. the 

 hydrogen tends to make the hot tungsten more electro- 

 positive with reference to the cold electrode. However, 

 further tests were inconclusive, the gas sometimes apparently 

 causing a small displacement in one direction and at other 

 times in another. All that we are prepared to state 

 definitely at present is that there is no effect of an otder of 

 magnitude exceeding that indicated by the figures given 

 above. 



Figf. 7. 



® pressure -157 MM. rig W x admitted to 



thoroughly c]em txue 



O " 000Z " " FILAMENT AND PLATE CLEAN 



FIL. CLEAN PLATE DIRTX 

 FlU/WD PLATE DIRTX. 



From time to time, and generally after the apparatus had 

 undergone some drastic treatment, displacements in the 

 characteristics of an entirely different order of magnitude 

 were observed. It seems fairly certain that these large 



