Mr. F. B. Fawcetfc on Standard High Resistances. 501 



more easily scratched off the glass with a needle than after- 

 wards. These effects seem to correspond with the toughening 

 of platinum wires by prolonged electrical heating in vacuo, 

 observed many years ago by Edison. 



Whatever may be the physical cause of the resistance- 

 change, it may be brought to an end by boiling the film 

 for several hours under reduced pressure in non-conducting 

 and chemically inactive oil ; and if after this the film be 

 hermetically sealed in its oil-bath, the vacuum being also 

 maintained, the resistance does not undergo further measure- 

 able change ; at any rate for seven months, the greatest range 

 of my experiments. This is shown by the results of tests on 

 three separate resistances, A, B, C, given in Table I. 



Table I. 

 Percentage Variation from Mean Value of Resistance. 





A. 



B. 



0. 





+0-0130 

 -0-0020 

 -0-0082 

 -0-0030 

 -00030 

 +00072 

 +0-0030 



-0-0012 

 +0-0055 

 +0-0043 

 -0 0086 

 -0-0012 



-0-0004 



+0-0005 



00000 









May ,, 



June and July 1898 







Mean resistance (ohms) 



97110 



161360 



502500 



Temperature coefficient, per 1 

 cent, variation per de- > 

 gree C , 



0-011 



0-010 



0012 



The variations given are in most cases the means of seven 

 or eight observations taken in the course of the corresponding 

 month. 



In the last line of the table is given the percentage increase 

 of resistance per degree centigrade rise of temperature. I 

 cannot find any published values of this quantity for an alloy 

 of gold and platinum, but 0*01 is smaller than one would 

 expect. To see whether the thickness of the film had any- 

 thing to do with it I deposited two films on the same piece of 

 glass, under precisely similar conditions, except that one was 

 deposited during a longer time than the other, and was there- 

 fore thicker. Table II. gives the values of the temperature- 

 coefficients (per cent, per degree) for the two films (Tube I.) 

 as well as those for a second pair (Tube 1L), prepared in the 



