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XLVI. The Photoelectric Effect on Thin Films of Platinum, 

 By J. Robinson, M.Sc, Ph.D., Lecturer in Physics at 

 East London College *. 



THE photoelectric current from thin films of platinum 

 does not increase uniformly with the thickness, but 

 varies *(" as shown in Curve A, fig. 2. This applies whether 

 the incident or emergent effect is considered ; i. e., whether 

 the light Falls on the platinum film before or after passing' 

 through the quartz plate on which the film is deposited. 

 For thicknesses below about 10 ~ 7 cm., i. e. from where the 

 current is a maximum, the emergent effect is larger than 

 the incident effect both as regards current and velocities ±. 

 An explanation of the difference in magnitude of the 

 incident and emergent currents was offered by Partzsch and 

 Hallwachs §, who showed that in the emergent case more 

 light is absorbed than in the incident case. This would offer 

 a full enough explanation if the magnitude of the effects 

 were exactly the same, but it would not explain the difference 

 in velocities. Hence doubt was cast on the result that the 

 velocities do differ. It has since been shown by Stuhlmann ||, 

 who used a method similar to the author's, that the emergent 

 velocity is larger than the incident velocity. 



Partzsch and Hallwachs' result, that more light is absorbed 

 in the emergent case, shows the necessity for further work to 

 find : — in the first place, what the asymmetry of the currents 

 is when unit quantity of light is absorbed in each case, and in 

 the second place, whether for very thin films the absorption 

 of lio-ht varies with the thickness in a way similar to that 

 of the current. 



In the experiments to be described, an attempt waa made 

 to get some information on these problems. It was con- 

 sidered advisable to make the measurements of the absorption 

 of light on the same films on which the photoelectric 

 measurements were made. 



ADEK is a nearly closed metallic cylinder standing 

 on a base D E. There is a small inlet of light at B. The 



* Communicated bv Prof. C. H. Lees F.R.S. 



t Robinson, Phil. Mag. xxv. p. 122 (1913); Werner. Ark. Mat. Ast. 

 Fys. viii. no. 27, p. 1 (1913) ; Stuhlmann & Compton. Phys. Rev. ser. 2. 

 ii. pp. 189 & 327 (1913). 



I Robinson, he. cit. 



§ Partzsch & Hallwachs, Ann. des Phys. xli. p. 247 (1913). 



(\ Phys. Rev. iv. p. 195 (1914). 



