142 Mi<s E. W. Hobbs on the Change in the 



Results. 



The mechanism of cathodic sputtering at low pressures is 

 probably similar to that of the production oE colloidal solu- 

 tions o£ metals in Bredig's method. The metallic particles 

 are molecular aggregates possessing a negative charge, which 

 they lose when, or shortly after, they strike the plate on 

 which the film is deposited. Hence a semi-stable layer of 

 particles is formed, having an electrical conductivity which 

 depends on the conditions during deposition and on the 

 thickness of the layer. In particular, Kohlschiitter and Noll 

 (loc. cit.) have shown that the nature of the residual gas 

 during discharge has a great influence on the fineness of the 

 particles deposited, and thus on the initial resistance of the 

 film and on the subsequent changes which occur. 



In the matter of adherence to the silica, films may vary 

 from a soft film easily rubbed off to one of a hard metallic 

 nature. An important factor in this connexion is the 

 cathode fall of potential, which controls the speed with 

 which the particles impinge on the plate, though the limits 

 of adherence vary with the nature and pressure of the 

 residual gas in the discharge-vessel. 



The Ageing of Films. 



It is well known that films deposited by cathodic sputtering 

 undergo a process of ageing, entailing a change in the 

 properties of the film. In 1898* Fawcett showed in this 

 laboratory that the change may continue for months, but 

 that it is hastened by rise of temperature. 



The change in condition after deposition is made apparent 

 by measuring the electrical resistance, which decreases with 

 time. An investigation was made of the effect of ageing 

 in vacuo on the electrical resistance of platinum and pal- 

 ladium films. The films were deposited at pressures varying 

 from *14 mm. to *4 mm. in air, and with a modified form of 

 apparatus from *5 mm. to *8 mm. in hydrogen ; the pump 

 was put on immediately discharge ceased, and resistance- 

 time measurements w r ere taken. 



Deposition in Air. 



A number of results obtained for platinum films deposited 

 in air (the case most fully investigated) are given in fig. 1, 



# Phil. Mag. xlvi. p. 500 ^1898). 



