ON PRACTICAL StANDAUDS FOR ELECTRICAL MEASUREMENTS. 



no 

 OJ 



taken as unit. The following table givea the values of the individual 

 tubes as found in 1903 and 1907 ; — - 



Except in the case of Tube P, where there is an apparent change of 

 3 to 4 parts in 100,000, the differences are negligible. 



Mr. Smith has also compared with the mercury tubes a large series of 

 wire standard resistances, including those made by Matthiessen and 

 Hockin for the B. A. Committee in 1865-67, and various other old 

 standards kindly lent to the Committee by their owners for the purpose. 

 The general conclusion is that all the original coils except D and E, 

 which are made of platinum, have changed appreciably since they were 

 constructed, though since 1888, during a period of twenty years, for which 

 the coils have been very carefully watched, the changes also in A, B, C, H, 

 and Flat have been small. F and G have, however, in the same period 

 changed considerably. 



Resistance at 16°-0 in terms of the orujinal B.A, unit (18G7). 



The mean resistance of 6 other platinum -silver coils, first measured in 

 1888, appears to have increased since that time by 14 parts in 100,000 ; 

 and .5 more platinum -silver coils, first measured in 1894-7, have now a 

 greater mean value by 8 parts in 100,000. 



It would appear also that in many of the variable coils the changes 

 have occurred mainly, if not entirely, at the soft-soldered joints, and with 

 a view of testing this the' Committee have authorised the Secretary to 

 open and examine one of the coils. 



A comparison, given in the Appendix, has also- been made of all the 

 Qianganin resistancea in the possession of the Standards Department of 



1908. D 



