6 7 J: Prof. J. Patterson on the Electrical 



deposited a good deal of gas is absorbed by them, but the 

 amount is so small that it would be impossible to detect it. 

 Lord Kelvin* has shown that a platinum plate which has 

 absorbed hydrogen is electro-positive to one that has not, 

 while if it has been kept in oxygen for some time it is electro- 

 negative to the standard. In the experiments on the contact- 

 difference of potential the film was positive to the platinum 

 cathode when the cathode was old, but negative when it was 

 polished, so that these experiments would not indicate whether 

 the gas was absorbed b}^ the film or not. It is not known just 

 what effect the gas in the depositing apparatus has on the 

 disintegration of the cathode by the discharge, but that there is 

 some effect is seen from the fact that aluminium which sputters 

 very little in air does so very freely in helium and argon. 



The condition of the surface of the cathode has also a 

 considerable effect on the rate of deposition, at least, of the 

 film. After the cathode had been used for a lono- time its 

 surface became dull and spongy in appearance. In this state 

 the rate of deposit was slower than when the surface was 

 freshly polished. 



It is most probable that part of the difference between the 

 measured resistance and the calculated resistance is due to 

 the manner in which the film is obtained. The film is formed 

 by particles of the metal striking on the glass surface and 

 adhering. These particles are more or less loosely connected, 

 and by heating they settle down into a more and more com- 

 pact mass; and consequently their resistance is decreased. 

 This is also seen from the fact that the rate at which the films 

 are deposited has a great effect on the condition of the film. 

 If the deposit is too rapid the film is powdery and does not 

 adhere to the glass. This phenomenon was very noticeable 

 in depositing the silver films, and Longden (loc. cit.) has also 

 noticed the same effect in platinum when using the Wehnelt 

 interrupter. It was also noticed in the bismuth films, and 

 that after a certain thickness was reached the surface lost 

 its metallic lustre and became dull and powdery in appearance. 



Another cause of the discrepancy would probably be the 

 impurity in the cathode, as small amounts of impurity affect 

 the resistance of the three metals very much. 



From the corpuscular theory of electrical conduction in 

 metals we have, if 



n be the number of corpuscles per unit volume, 

 e the charge on a corpuscle and m its mass, 

 X the mean free path, 

 c the initial velocity, 



* ' Nature/ vol. xxiii. p. 567. 



