Properties of Thin Films of Platinum. 127 



Discussion. 



10. If we wish to explain the variation of the magnitude 

 of the photo-current with the thickness of film, we must also 

 take into account some results recently given by Dyke "* on 

 the variation of the photo velocities with the film thickness. 

 His results are given only for the incident effect. Velocity 

 measurements involve so many difficulties that no regular 

 curves were obtained, but a general statement of Dyke's 

 results is that for films thinner than 10~ 7 cm. the velocities 

 are high, and for thicker films much lower. This thickness 

 10 ~ 7 cm. is also a critical thickness for the velocities, which 

 undergo a sudden decrease here. 



We will consider the merits of certain possible explana- 

 tions of these photoelectric phenomena at the thickness of 

 film 10 ~ 7 cm. 



(a) It was shown by Paterson -j- that the specific resistance 

 of thin films of platinum depends on the thickness. As the 

 thickness diminishes from 10 " 6 cm. the specific resistance 

 remains fairly constant till the thickness 10 " 7 cm. is reached, 

 when it begins to increase rapidly. As this sudden change 

 in specific resistance occurs for the same thickness as the 

 change in the photoelectric effects, it might be suggested 

 that the high resistance of films thinner than 10" 7 cm. 

 accounts for the small values of the photo-currents observed. 

 This is, however, improbable, for when measurements of the 

 current were made by the leakage method no lag of the 

 electrometer needle was observed, and such would certainly 

 have been observed if the films had an enormously high 

 resistance. The readings of the currents were always steady 

 and very consistent for the thinnest films. 



(b) Again, no satisfactory explanation can be found by 

 considering the films to be discontinuous. Such discon- 

 tinuities obviously will exist for very thin films, but as long- 

 as there is some metal on the quartz plate, the film will 

 conduct if there is no break right across it of more than 

 about *001 inch in width. Wood J has recently shown that 

 conduction can take place between two metals kept at this 

 distance apart. Discontinuities might exist so that the film 

 appears something like a draughts-board. As more metal 

 is deposited the empty spaces left get gradually filled up. 

 Hence the area of metal exposed to the light might alter 

 with the time of deposit. It is, however, difficult to see how 

 an explanation can be reached along this direction. 



* Phys. -Rev. xxxiv. p. 459 (1912). 



t Loc. cit. 



X Phil. Mag. Aug. 1912, p. 316. 



