Properties of Thin Films of Platinum. 129 



If primary electrons can produce secondaries by collision, 

 then we ought to expect a diminution in the velocity of the 

 electrons at 10~ 7 cm., which is what Dyke found. 



The fact that the critical thickness is the same for the 

 slow electrons produced by the arc and the quicker electrons 

 due to the spark, is also in harmony with this view, for the 

 mean free path of the electrons does not vary much with 

 their velocity. 



Again, for films thinner than the mean free path, the 

 photo-currents will not be influenced by the absorption of 

 electrons, which can only take place when the electrons 

 come near to the molecules. This explains why the ratio 



"F 



Y currents is constant for thicknesses up to 10 " 7 cm. For 



thicker films, absorption will take place so that the ratio 



Y begins to diminish. 



11. Minimum Energy for Ionization. 



It is necessary to consider whether slow photoelectrons 

 have sufficient energy to ionize molecules of platinum. 

 Estimates of the minimum amount of energy required to 

 ionize a molecule have been given by different writers. 

 Hughes * estimates this quantity to be that of an electron 

 of 8 volts velocity for oxygen. 



The similarity of the curves for the arc and the spark in 

 fig. 6 suggests that their characteristics have the same origin. 

 There is no doubt that the 50 volt electrons produced by the 

 light of the spark can ionize, and as all the other facts agree 

 with this ionization theory, we conclude that it is possible 

 that the slow photoelectrons due to the arc also have sufficient 

 energy to ionize. 



The method employed by Hughes to calculate the minimum 

 amount of energy required to ionize a molecule of oxygen is 

 interesting, and it can readily be applied to the present 

 problem. He found that to ionize oxygen by light, the 

 wave-length must not be longer than X 1350. He showed 

 how to represent the velocity of photoelectrons as a function 

 of the frequency of the light. If V is the velocity in volts, 

 and n is the frequency of the light, 



Y = kn-Y , 



where h and V are constants for each substance. As 

 \ 1350 is the wave-length limit for oxygen, an electron 

 emitted by light of this wave-length would just emerge from 



* Loc. cit. 

 Phil. Mag. S. 6. Vol. 25. No. 145. Jan. 1913. K 



