156 



EEPORT — 



for the other coil, 

 results : — 



The four measurements of resistance gave the following 



A. Mr. Glazebrook in March, 1894. 



B. Board of Trade in Nov., 1896. 



C. Board of Trade in Aug., 1897. 



D. Mr. Glazebrook in Dec, 1897. 



Resistance = 99926 ohms at 149° C. 

 = 9-993213 „ 14-91° C, 

 = 1000775 „ 19 3° C. 



= 9-9896 „ 13-9° C. 



From these we get the following values for the temperature coetti- 

 cient : — 



Here again the same two sets of tests, viz., tests B and C, and tests 

 D and C, give values for the temperature coefficient very nearly equal to 

 those I obtain for the same range of temperatures. 



Since for both coils the temperature coefficients that I obtain agree 

 with those calculated from the three last measurements of resistance — 

 namely, the two measurements by the Board of Trade and Mr. Glaze- 

 brook's last test — these experiments seem to show that the coils have not 

 changed since 1896, but that the resistances as measured in ISO-t were a 

 little lower than those that would now be obtained at the same tempera- 

 tures. 



This conclusion may be better illustrated by calculating what would 

 be the resistances at the temperatures of the various tests, on the assump- 

 tion that the coefficients I obtain are correct, and that Mr. Glazebrook's 

 last test (in December, 1S97) is correct. We then get the following : — 



Temperature of test 



Coil No. 3,873 



Coil No. 3,874 



14-8° C. 

 14-86° C. 

 19-3° C. 

 13-9° C. 

 14-9° C. 

 14-91° C. 

 19-3° C. 

 13-9° C. 



Thus we see that the 1894 measurements (A) are too low by as much 

 as 7 parts in 100,000 in the case of coil No. 3873, and 6 parts in 100,000 

 in the case of No. 3874. In the case of the other two measurements the 

 calculated results only differ from the observed results by 1 or 1*5 parts 

 in 100,000. 



These experiments were carried out in the laboratory of the Central 

 Technical College, South Kensington, and I am much indebted to Pro- 

 fessor Ayrton and Mr. T. Mather for their valuable guidance and advice. 



