216 



REPORT — 189/ 



with the Lorenz apparatus, and the data of the second instrument are 



contained in the following table. 



Kesistance of suspended coil .... 



„ coil and suspension 



Periodic time of complete swing 



Scale distance actually used 



in divisions at actual scale distance 



Deflection 



used 

 Deflection in divisions at scale distance equal to f 204 



2,000 scale divisions \ 35-8 



1^9 ohms. 

 6-75 „ 

 7-6 seconds. 

 f 1412 millimetres, 

 i 1340 scale divisions-, 

 i 137 per micro-ampere.. 

 \ 23-8 „ micro-volt. 



micro-ampere, 

 micro-volt. 



The resistance coils used were those previously employed in the Cardiflf 

 determination of the ohm {vide Report of the Committee on Electrical 

 Standards, Appendices IT. and III., 1894). They have been tested once 

 by Mr. Glazebrook, and twice by the kindness of Major Cardew in the 

 Board of Ti'ade Electric Standardising Laboratory, with the following 

 results : — 



The coils Nos. 3,873 and 3,874 were stated by the makers, Messrs. 

 Nalder, to be wound with platinum silver wire, and the two others, Nos. 

 4,274 and 4,275, with manganin. 



In the following table are given the temperature coefficients as- 

 Bupplied originally by the makers, and as calculated from the tests A and 

 C, and B and C. 



Temperature Coefficients of Resistance per \° C. 



These figures show that a redetermination of the temperature co- 

 efficients, which we are now carrying out, is necessary. 



Fortunately the last set of determinations of the resistance of these four 

 coils was carried out at Westminster, within a fortnight of the completion of 

 our absolute measurements, and we are much indebted to Major Cardew 

 for his kind promptness in the matter. The temperatures of these 1897 

 Board of Trade measurements were so nearly those of the coils during 

 our final absolute determinations, which were from 18°'8 to 19°-4 C, as 



