Properties of Thin Films of Platinum. 



121 



ordinates. A smooth curve drawn through the points is 

 found to pass as nearly as possible through the point 1, 1 



Hence we find that the ratios y currents and -y velocities 



are unity, for the same thickness of film. 



6. Actual Magnitudes of the Photo- Currents. 



No attempt was made in the experiments just described to 

 compare the actual magnitudes of the photo-currents for 

 different film thicknesses, but it was noticed that these 

 currents were certainly not proportional to the thickness of 

 the film. Attention was now directed to this point. It was 

 found to be unnecessary to correct for any variations in the 

 intensity of the mercury lamp used, as this formed a very 

 constant source of ultra-violet light. From a large number 

 of experiments the same general results were obtained. A 

 typical set is given in Table III. The emergent and incident 

 currents are both plotted against the thickness of the film in 

 fig. 4 (p. 122). 





Table III. 





Thickness 



of 



Film. 



Photo-Currents. 



E 

 I • 



Emergent 

 E. 



Incident 

 I. 



20 seconds 



5-4 



G-5 



11-5 



378 



230 



198 



3-8 

 47 

 8-5 



310 



197 



180 



121 

 1-22 



1-27 

 1-21 

 1-16 

 1-10 



40 „ 



60 „ 



100 



140 „ 



180 „ 





The most striking point about these results is the sudden 

 increase in current at a certain thickness of film. An esti- 

 mate of this thickness was obtained in the following way : — 



After the photoelectric measurements were completed, the 

 discharge was passed for a definite time to obtain a thicker 

 film for which the specific resistance is known fairly accu- 

 rately *. The resistance of this thick film was measured and 

 its thickness calculated, making use of Paterson's values. 

 From the relative times of deposit, the thickness of the film 



* Paterson, Phil. Mag. iv. p. 652 (1902). 



