﻿Properties of Thin Metal Films. 



543 



It has been found that the photoelectric properties of 

 metal films vary much under different circumstances, and 

 depend especially on whether they have been in the atmo- 

 sphere or not. Consequently, in the experiments described 

 in the following pages, care was taken that the films were 

 deposited at a low pressure in the tube in which measure- 

 ments were to be made, and immediately after depositing, 

 as high a vacuum as possible was obtained, and maintained 

 during the course of some of the experiments. Measure- 

 ments were made of the photoelectric velocities and currents 

 for incident and emergent light. 



Description of the Apparatus. 



The quartz plate A (1^ mm. thick) was attached to a 

 thick brass rod B, which was fixed by sealing-wax through 



Fig. 1. 



an amber plug C, which in its turn was fixed into one part 

 D of a ground joint S. By means of this ground joint the 

 quartz plate A could be made to take up any angle with 

 regard to the direction of the light through the tube EE. 

 This tube was narrow so as to admit only a thin beam of 

 light. The quartz plate F closed the tube E. G was a 

 platinum electrode attached rigidly to a rod, at the other end 

 of which was an iron ring H. By applying an electro- 

 magnet from outside to this iron ring, the system HG could 

 be ii^oved along the tube KK. Before the quartz plate A 

 and the platinum electrode G were inserted, the walls of the 

 tube KK were coated thickly with platinum by stretching 

 a platinum wire through the tube, and passing a current 

 between it as cathode, and a, wire in one of the side fcnbes 



