Temperature upon Photo-electric Effects in High Vacuum. 205 



of the liquid upon the metal. With copper, gold, and the 

 other electro-negative metals, this double- layer constitutes 

 a powerful retarding electric field through which the electrons 

 detached by resonance from the atoms must pass in order to 

 escape. This field is so strong that practically all of the 

 electrons which are detached, are pulled back to the surface 

 of a noble metal so long as it is in air. With the highly 

 electro-positive metals, on the other hand, this field is relatively 

 weak, so that, though the velocities of projection and the 

 number of electrons detached are much smaller than before, 

 a larger number, nevertheless, escape. In a word, in air 

 the effect of the double-layer entirely masks the true photo- 

 electric effect. In a vacuum the double-layer disappears 

 and the rate of discharge under the influence of the ultra- 

 violet light becomes, for the first time, a measure of the 

 influence of the light in detaching the electrons from the 

 atoms of the metal. These experiments seem, then, to render 

 quite untenable the view brought forward by Ramsay and 

 Spencer that the order of affinity of the atoms of the 

 elements for the electrons which they part with in the process 

 of disintegration (under the influence of ultra-violet light) is 

 the same as the order which they exhibit electro-chemical 

 attractions. The two orders are approximately the same 

 when the photo-electric experiments are conducted in air 

 merely because the strength of the double-layer, which is of 

 course determined by electro-chemical attractions, exerts in 

 most cases a controlling influence upon the photo-electric 

 phenomenon. Equally untenable appears to us the view 

 that the order in which the metals exhibit photo-electric 

 sensitiveness bears any relation to the order in which they 

 are undergoing spontaneous disintegration. 



9. The Relative Positive Potentials acquired by Different 

 Metals. — Since, according to the above considerations, the 

 determination of the relative positive potentials acquired by 

 different metals under the influence of ultra-violet light gives 

 us direct information regarding the relative velocities of the 

 electrons within the atoms of the different elements, it is a 

 matter of considerable interest that these potentials be 

 correctly determined. The values of the positive potentials 

 acquired by aluminium and platinum in Lenard's experiments 

 were three and two volts * respectively. The value given in 

 Table III. for aluminium is but *738 volt. This difference 

 might be due to the difference in the nature of our sources, 

 and also to the fact that we have not eliminated entirely 



* Lenard, Ann. d. PJn/s, viii. p. 180 (1902). 



