104 



Prof. A. LI. Hughes on the Velocities of 



The method of experiment was as follows: — When there is 

 no field between the alloy and E, the electrons leave the 

 illuminated surface in straight lines in all directions. When 

 a field is established, the paths of the electrons are carved 

 more or less towards the electrode E which now receives a 

 greater current. A sufficiently large field will pull all the 

 electrons into E. The number reaching E is a function 



(1) of the potential difference between it and the alloy, and 



(2) of the velocity and direction of emission of the electrons. 

 There are two distributions of electrons to consider, a direction 

 distribution and a velocity distribution. If the photo-electrons 

 liberated in the selective effect and those liberated in the 

 normal effect had identical velocity distributions and direction 

 distributions, then one would obtain exactly similar curves 

 connecting the charge received by the electrode E and the 

 potential applied to the alloy. 



The current obtained with an accelerating potential of 

 404 volts is arbitrarily taken to be equal to unity, for both 

 effects. The results are given in Table I. Some experiments 





Table 



I. 





Accelerating 



Photo-electric currents. 



Ratio 



S 



Selective effect 



Normal effect 



potential. 



(S). 



(N). 



N" 



404 volts. 



l-00( = 295xl0" 12 amp.) 



1-00 ( = 11-4X10~ 12 amp.) 



1-00 



302 



•962 



•910 



1-06 



129 



•723 



'628 



1-15 



87 



•627 



•528 



1-19 



44 



•463 



•320 



1-26 



18 



•307 



•212 



1-45 



12-8 



•261 



•173 



1-51 



8-0 



•209 



•133 



1-57 



39 



115 



•0712 



1-61 



2-00 



•0522 



•0291 



1-79 



1-46 



•0308 



•0160 



1-93 



1-00 



•0116 



•0055 



2-07 



were carried out with the same aperture at the lens for both 

 planes of polarization. In these cases, the selective effect 

 with 404 volts accelerating potential was 25*9 times larger 

 than the normal effect. In other cases, the lens was 

 stopped down to one twenty-sixth part of its area when the 

 selective effect was being measured, so as to make the selective 

 and normal effects of the same order of magnitude. The 



