502 Mr. F. B. Fawcett on Standard High Resistances. 



same way. The column headed approximate thickness of 

 film was calculated from the resistance, length, and breadth of 

 the film on the assumption that the specific resistance was the 

 same for all the films. The thickness is expressed in arbitrary 

 units, but if the specific resistance of the films is anywhere 

 near that of platinum-silver the numbers in the column give 

 the thickness of the film in million ths of a centimetre to the 

 same degree of approximation. 





Table II. 







Approximate 

 thickness. 



Temperature- 

 coefficient. 



Tube I. 

 Thin 



2 



54 



1 

 99 



i 

 00085 



00130 



0-0028 

 00153 



Thick 



Tube II. 

 Thin 



Thick 



The results clearly show how the temperature-coefficient 

 decrease? as the thickness of the film decreases. In fact I do 

 not think it impossible that films may be made of nega- 

 tive, and therefore also of zero temperature-coefficients ; 

 one extremely thin film having show T n signs of a negative 

 coefficient. Its resistance became infinite, however, before I 

 was able to test it perfectly, and 1 am thus unable at present 

 to speak decidedly on the point. I hope shortly to investigate 

 the whole question of the temperature-coefficients of films. 



It only remains to describe the manner in which the re- 

 sistance of a film may be adjusted to a definite value. 



julqjuulijlQ 



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The film is deposited to such a thickness that its resistance 

 is far below the required value. It is then heated in oil, 

 under reduced pressure, to render it electrically stable (as 

 explained above). In this condition it is scratched with a 

 needle in the manner shown in the figure, so that it becomes 

 a long zigzag narrow strip, and the scratching process is con- 

 tinued until its resistance has arrived at the required value. 



Summary. 

 In conclusion I may point out that metal films treated in 



