128 Dr. J. Robinson on the Photoelectric 



(c) The effects do not seem to be due to gas layers at the 

 surface of the films. In most of the experiments when the 

 greatest precautions were taken, there were most probably 

 no such gas films. Had there been any they would most 

 probably have influenced thin films and thick films alike. 



(d) Possibly platinum films can only absorb gas when 

 their thickness is greater than 10~ 7 cm. If we make this 

 assumption, and assume besides that the photoelectric effect 

 is due to absorbed gas as well as to the metal, then the 

 sudden increase in current can be explained. There would 

 also be many more molecules about in proportion to the 

 thickness at 10 ~ 7 cm. than for thinner films, so that the 

 photoelectrons would have their energy reduced by col- 

 lision. Hence we might account for a decrease in velocity 

 also. But although the photoelectric effects may be in 

 harmony with this view, it is difficult to see why the 

 specific resistance should diminish so much when gas is 

 absorbed. 



(e) There may be some alteration in the power of films to 

 absorb light at 10 " 7 cm. We might suppose that there is a 

 sudden increase in the absorption of light and so account for 

 the sudden rise in current, but such an assumption would 

 not be in harmony with the diminution of velocity, for it is 

 generally supposed that the photo velocities are indepen- 

 dent of the intensity of light, and of the amount of light 

 absorbed. 



(/) An explanation which is in harmony with all the 

 £; its known at present is the following : — 



Let us suppose that the photoelectrons have the power of 

 ionizing molecules of platinum by colliding with them. Then 

 there will be ' two kinds of electrons in the photoelectric 

 effect of thick metals: "primary," which are produced by 

 light falling on molecules, and " secondary," which are pro- 

 duced by the collision of the primary electrons and mole- 

 cules. To have both kinds of electrons in a film the 

 thickness must be greater than a certain quantity X, which 

 we may call the mean free path of the electrons. For films 

 thinner than X we have only primary electrons, and for 

 those thicker than X, both primary and secondary. This 

 accounts for the sudden increase in photo-current if the 

 thickness at which it takes place is equal to the mean free 

 path of electrons in platinum. An estimate of this quantity 

 was given by Paterson * from measurements of the Hall 

 effect, and he found X of the order of 10~ 7 cm., which is the 

 critical film thickness for the photoelectric phenomena. 



* Loc. cit. 



