Properties of Thin Films of Platinum. 117 



the walls of the vessel were coated with platinum before 

 commencing the actual experiments, and the potential of the 

 walls controlled by the wire M which was made to touch the 

 platinized walls. The quartz plate A was 1*5 mm. thick, 

 and 2*5 cm. long by 2 cm. wide. It was attached rigidly 

 to the rod B, insulated by an amber plug C, and was 

 capable of rotation about B as axis by the ground joint S. 

 To deposit a film on the quartz plate A, the platinum 

 electrode G was moved up to a fixed distance of 1 inch from 

 it, and a discharge passed from an induction-coil, using Gr as 

 cathode and an insulated wire at N as anode. This was 

 done at a low pressure, and in most cases before depositing 

 the discharge was passed for a few hours with Gr far 

 removed from the quartz plate A which was placed with 

 its plane parallel to the axis of the tube KK. This pre- 

 caution was taken so as to get rid of any impurities on the 

 surface of the platinum electrode, and to get it into the 

 condition that it has stopped giving out gas *. The gas 

 given out by the platinum was pumped away by a Gaede 

 pump, and only when the amount given out per minute w r as 

 small was a film deposited. 



After depositing a film, a liquid air vacuum was obtained 

 and measurements made. Then the film was thickened 

 slightly and the measurements repeated, and so on till the 

 film was so thick that the emergent effect was small. 



Electrical contact with the film was made by first of all 

 depositing a thick film of platinum on one end of the quartz 

 plate. Tinfoil was laid over part of this, and the plate 

 held firmly in a clamp which was soldered to the rod B. 

 Platinum was deposited over this whole system. In some 

 experiments the edge of the plate was silvered instead of 

 being thickly coated with platinum. 



The tube EE to admit the light was narrow so that light 

 fell only on the centre of the film. 



The rod B was connected in the ordinary way to one pair 

 of quadrants of a Dolezalek electrometer. Measurements 

 of the velocities of the electrons were made by earthing the 

 walls of the vessel and finding the maximum potential 

 acquired by the film. In some cases the velocities were 

 also measured by finding the retarding potential that must 

 be applied to the walls to prevent a photoelectric leak. 

 The currents were measured by charging the walls to 

 a potential of 4- 60 volts. The quadrants were joined 

 through a high resistance of xylol and alcohol of 10 10 ohms t, 



* Hodgson, Phys. Zeit. 1912, p. 595. 



t Campbell, Phil. Mag. Aug. 1911, p. 301. 



