Photoelectric Properties of Thin Films of Platinum. 115 



in (2), so that all the quantities required by (1) in order to 

 give H in terms of the measured quantity Q are known. Q 

 has of course to be corrected for radiation from the calori- 

 meter, but this presents no difficulty. The constant B 

 occurring in Regnault's correction formula is correctly 

 determined by his method, and is not affected by the flow. 

 It is quite easy to extend the above argument to the case 

 where the connecting pipe really consists of two pipes of 

 different materials, as in some of Regnault/s experiments. 



Physical Laboratory, 



The University, Sheffield. 

 Oct. 18, 1912. 



XII. The Photoelectric Properties of Thin Films of 

 Platinum. — Part II. By J. Robinson, M.Sc, Ph.D.* 



Introduction. 



V 

 to 



1. TT was shown by Stulihnann *" that the photoelectric 

 JL effect of thin films of different metals deposited on 

 quartz depends on the thickness of the film, and on whether 

 the film is on the side of the quartz facing the light (incident 

 effect), or on the side away from the light (emergent effect). 

 He measured the ionization currents in air, and found that 



when the films are thin enough the ratio — — r-r- — ( - r ) 



° incident \ 1 / 



currents is greater than unity, and for thicker films less than 



unity. For thin films the ratio is constant and equal to 1*14 



for platinum. 



In a former paper by the writer J it was shown that this 



dissymmetry observed by Stuhlmann can be separated into 



two quite distinct effects, as regards (1) the velocities of the 



electrons emitted, and (2) the actual numbers of electrons 



emitted. The films were deposited at a low pressure, and 



then a liquid-air vacuum was established and measurements 



made as quickly as possible afterwards. In this way it was 



hoped that true values for the velocities would be obtained. 



For both the photo-currents and the maximum velocities of 



the electrons, similar curves to that of Stuhlmann for the 



ratios ( -==. ) ionizations were obtained as the thickness of the 



(!)' 



* Communicated by the Author. An account of part of this paper 

 was read at the British Association Meeting at Dundee, September, 

 1912. 



t Phil. Mag. Aug. 1910, p. 331. 



I Phil. Mag. April 1912, p. 542. 



I 2 



