Properties of Thin Metal Films. 653 



Prof. Thomson * has shown that this variation of the re- 

 sistance with the thickness may be explained by the cor- 

 puscular theory of electric conduction in metals. According 

 to this theory the current is carried by negatively charged 

 particles moving with a definite velocity under an applied 

 E.M.F. Thes^ particles have the properties of a perfect gas, 

 consequently they have a mean free path. When this mean 

 free path becomes comparable with the thickness of the film 

 it will be decreased, and consequently the resistance of the 

 film will be increased. (This theory will be discussed more 

 fully later.) 



If, then, the point at which the resistance of the film 

 becomes abnormal can be obtained, this thickness would be 

 an approximate value for X the mean free path. 



The mean free path of the corpuscles in a number of metals 

 has been determined by the author (Phil. Mag. June 1902) 

 by measuring the change of resistance in the metal, produced 

 by a transverse magnetic field. 



It was hoped that the present investigation would furnish 

 another and entirely different method of determining the 

 mean free path. 



Description of Apparatus. 



The metallic films, which will be called simply films through- 

 out the paper., were obtained by sputtering from a cathode in 

 vacuo on glass strips. These strips were of ordinary glass 

 about 1 mm. thick, 12 mm. wide, and from 4 to 7 cm. long. 

 To ensure good contact between the electrodes and the film 

 the ends of the strips were silvered for about 1 cm. with one 

 of the ordinary silvering solutions. Owing to the surface- 

 tension of the silvering solution the thickness of the silver 

 deposit gradually increased from zero up to its full value. 

 This ensured good contact between the silver and the film. 

 The surface-tension also made the line of contact between the 

 film and the silver curve at the edges, and thus made it 

 difficult to measure the length accurately. This defect was 

 overcome in the later experiments by cutting glass plates 

 about 4 cm. wide and 20 cm. long. The sides were silvered 

 as before, and then the plate was cut into strips of the desired 

 width. Fig. 1 (p. 650) shows the plate with the sides silvered, 

 and the dotted lines denote where it was cut. The two end 

 pieces were rejected, and the others were all of a uniform 

 length and the line of contact between film and silver was 



* Proc Camb. Phil. Soc. vol. xi. pt 2, p. 119 (1901;. 



