ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



43 



The difference from the values given by Lord Rayleigh is 3 parts in 

 100,000, which is less than the probable error of the observations. We 

 conclude, therefore, that the determination of Lord Rayleigh and Mrs. 

 .Sidgwick in 1881 is in excellent agreement with that made at the 

 N^ational Physical Laboratory in 1908, and this latter has already been 

 shown to agree with that made by Dr. Glazebrook in 1888. 



The following is now a very useful sumtnary. The values of the 

 coils in centimetres of mercury in 1881, 1888, and 1908 are given in 

 Table VIII. 



Table VIIL 



Giving the Values at 16°'0 0. of certain Coils in cm. of Mercury in 1881, 1888, 

 and 1908 obtained from comparisons %oith Mercury Standards, 



The preceding comparison strengthens the conclusions already arrived 

 at respecting the most constant coils. From Table VIIL, D and E have 

 apparently kept constant in resistance since 1881, while H appears to 

 have remained constant since 1888. 



It is of some importance to note that in 1892 the ratio of the B.A. 

 unit to the ohm was accepted as being 



1 ohm = 1-01358 B.A. unit, 



this being based on the values 



100 cm. mercury = 0-9535 B.A. unit. 

 106-3 cm. mercury =1 ohm. 



Other Platinum -Silver and Gold- Silver Coils. 



In addition to the platinum-silver coils, V, G, H, and Flat, originally 

 constructed to represent the B.A. unit at a particular temperature, there 

 are three other platinum-silver coils, numbered 3715 (Nalder Brothers) 

 and 269 and 270 (Elliott Brothers), made to represent the ohm = l-01358 

 B.A. unit. There are also two 10-ohm platinum-silver coils, numbered 

 288 and 289 (Elliott Brothers). All these coils are the property of the 

 Association, and they were extensively used from 1888 to 1903 for the 

 standardising of other coils. From the results of observations recorded 

 in the Report for 1903 it appears that from 1894 to 1903 Nos. 3715 and 



