Properties of Thin Metal Films. 667 



film for some time by the above means the resistance was 

 reduced to 12*06 ohms per cm. square. The resistance was 

 not further reduced by heating with the current. Grlass, 

 however, would not stand the temperature to which the films 

 were raised, so mica was used instead. The films deposited 

 on mica were heated in vacuo by the electric current. They 

 were found to stand a current-density from l'O to 2*0 X 10° 

 amperes for a few minutes, and then only when the current 

 was gradually increased. Putting the full current on at once 

 destroyed the films instantly. 



Two films were deposited on mica with resistances of 

 lo'SO and 20*24 ohms respectively. They were heated in 

 vacuo by the electric current, and the following results were 

 obtained : — 



1 ampere for 8 hours reduced resistance of 1st from lo"80 to 5*18 ohms. 



1*2 amperes „ 20 „ more, resistance became 14*89 ohms. 



04 ampere „ 8 „ reduced resistance of 2nd from 2024 to 15*77 ohms. 



0-6 





„20 „ 



more, resistance became 8*88 ohms 



0-6 



» 



,24 „ 



n ii n b'J® „ 



The first film had a resistance of 1*79 ohms per cm. square 

 when it w*as least, and. the second 2*62 ohms per cm. square. 

 The thickness of these films was not measured, but the specific 

 resistance would be about 30 X 10~ 6 ohms per cm. cube. 



These results show that the final specific resistance of the 

 films depends on the treatment that they receive. It was 

 impossible to reduce the resistance of the films to that for 

 ordinary platinum. 



These results also show that heating the films with the 

 electric current has a much greater effect than heating in an 

 air-bath. This is most likely due to the higher temperature 

 to which the films are raised in the former case. 



The temperature-coefficient of a number of the films was 

 also measured. They were heated in an air-bath to about 

 50° C. or cooled down to 0° C, the temperature of the room 

 being about 15° C. The temperature-coefficients for a number 

 of the films are recorded in Table VII. 



Table VII. — Temperature-Coefficient of Platinum Films. 



dumber of 



Thickness in 



Eesistance per 



Temperature- 



film. 



cm. 



cm. square. 



coefficient. 



66 



5-92 XIO"* 3 



14-57 



0-00040 



64 



1-85 „ 



4231 



0005 



77 



0-46 „ 



242-1 



0-00033 



81 



0-38 „ 



408-3 



00024 



82 



0*26 „ 



11855 



000027 



70 



0--1 ., 



2598-4 



000011 



