130 



REPORT 1894. 



APPENDIX IV. 



ComjMrison of certain Ohm-Standards of the Board of Trade. 



By J. Rennie. 



In the accompanying table are given the results of comparisons which 

 were made on May 29 and 30, 1894, at the Cavendish Laboratory, between 

 the three unit coils : — 



Elliott's No. 261, 



Elliott's No. 263, 



Nalder'sNo. 3876, 



belonging to the Electrical Department of the Board of Trade, and the 

 B.A. standards, Flat, F, G, and H. 



The bridge was of the Carey Foster pattern, constructed for the 

 Department by Nalder Bros, and Co., and the slide wire used was the 

 one marked B, having a value of -000,050,9 ohm per division. 



A 100-ohm coil, Elliott's No. 291, was placed in parallel with the 

 Board of Trade coil for each comparison, this being effected by a large 

 mercury-in-paraffin bath. 



Temperatures were measured by a mercury-in-glass thermometer, which 

 had been standardised at Kew, 



* The chart referred to, for No. 261 and No. 263 coils, is one supplied for these 

 coils by Mr. Glazebrook, and is dated March 1892. The chart referred to la the case 

 of No. 3876 was constructed from comparisons made by Nalder Brothers between it 

 and their 'master coil,' No. 3717. The coils Nos. 263 and 261 were compared on 

 May 29, 1894, before beginning the above-mentioned series of comparisons. Thej' 

 were found exactly equal, when the temperatures were — No. 263, 12°-65 C. ; 

 No. 261, 12°-62 C. The chart values at these temperatures are— No. 263, 0-999175 ; 

 No. 261, 0-999156 ; showing a difference of 19x10"^ ohms. The corresponding dif- 

 ferences deduced from the above table are — from Flat, ISxlO'i* ohms; from 

 F, 8 X 10"^ ohms ; from G, 9 x 10"^ ohms. The comparison No. 261 — H is omitted, as 

 the difference obtained was obviously much too large, and must have been caused 

 by some undetected interference. It is evident from the eleven results given in the 

 table that the difference between the coils Nos. 263 and 261 as deduced from 

 comparison with H must be something like 10 x 10"^ ohms. [Note added October 5, 

 1894.] 



