﻿Properties of Thin Metal Films. 



549 



the case. In the following table the ratio of the maximum 

 currents is shown in relation to the thickness of the film. 



Table III. 



Kelative thickness 

 of film. 



Maximum current. 



A 

 B* 



Emergent. 

 A. 



Incident. 



5 minutes 



40 

 18 

 SO 



18 



31 

 16 



27 

 59 



1-29 

 112 

 111 

 031 



8 „ 



18 „ 



55 ,, 





Discussion of Results . 



Velocities. — Fig. 2 shows that for thin films the velocities 

 of the electrons for emergent light are greater than those 

 for incident light, and that for thick films the reverse is the 

 case. The velocities measured were the maximum velocities 

 in each case, which, according to Ladenburg *, refer to the 

 shortest wave-lengths of light used. Thus one particular 

 wave-length of light produces quicker electrons for emergent 

 than for incident light, when thin films are used, and 

 vice versa for thick films. 



It was thought that possibly the results for thick films 

 might be due to some selective absorption of light by them. 

 Thus if the shortest wave-lengths are absorbed completely 

 before the light has penetrated far into the film, then the 

 velocities of the emergent electrons will be smaller than 

 those of the incident. It was not found possible to sub- 

 stantiate this possibility by taking photographs of the spectrum 

 of the lamp used. A Hilger quartz spectroscope was used 

 and photographs taken : — 



1st. With nothing between the lamp and the spectroscope. 

 2nd. With a film of 18 minutes deposit before the slit, 

 ord. ,, ,, oD j, „ 



In each case a number of exposures of different lengths 

 of time were made, and although a general absorption of 

 light by the films was observed, yet it was not possible to 

 detect any selective absorption. The shortest wave-lengths 



* riojs. Zcu'L 1907, p. 590. 



