Properties of Thin Metal Films. 673 



connexion between the current through the film and the 

 positive leak. 



These experiments were not pursued further since they show 

 that owing to the large radiating surface, the film would not 

 stand temperatures high enough to give negative radiation. 



Change of Phase of the Light Refected from a 

 Platinum Film. 



Duplicates of the platinum films given in Table VI. were 

 deposited on mica thin enough to give interference-bands in" 

 the spectroscope. One halt of the mica was covered with 

 another thin sheet so that no metal could be deposited 

 on that half while it deposited on the other part. This mica 

 was then placed before the slit in a collimator o£ a spectro- 

 scope with the metal side away from the aperture. Two sets 

 of bands were thus obtained, one due to the reflexion from 

 the mica-air surface, and the other from the mica-metal sur- 

 face. The displacement of the bands due to the change of 

 phase at the mica-metal surface was towards the red end 

 of the spectrum. 



With the very thin films the displacement was very small, 

 but increased rapidly with the thickness up to its full value 

 and then became fairly constant. 



Theoretical Considerations. 



The films of the three metals — platinum, silver, and bis- 

 muth — deposited in vacuo by the cathode discharge, all have 

 their resistance considerably higher than the value calculated 

 from the specific resistance of the metal. A number of 

 films were examined under the microscope but no discontinuity 

 could be observed. Reducing the width of the film did not 

 affect its resistance per cm. square, and, moreover, the effect 

 of the electric current on the platinum films would tend to 

 increase rather than to diminish any discontinuity that might 

 exist. The experiments would show that the thickness was 

 fairly uniform, and would not vary by more than a small 

 fraction of the average thickness. 



femall quantities of many different gases are present in a 

 vacuum-tube, hydrogen and mercury vapour being always 

 present unless very special precautions are taken to get rid 

 of them. Most metals absorb gases to a greater or less 

 extent, platinum especially absorbs a good deal of hydrogen. 

 It may be that from the manner in which the films are 



Phil .Vug. 8. 6. Vol. 4. No. 24. Dec 1902, 2 Y 



