STANDARDS OF RESISTANCE, 1865-1932 419 



During 1879-81 a very careful comparison of the coils was made by 

 Dr. Fleming. It was clear that their relative values had changed appreciably 

 since 1867, and he adopted as a definition of the B.A. unit the mean value 

 of the resistance of all the coils at the temperatures at which they were 

 originally said to be correct. For his comparisons he employed Carey 

 Foster's method, and devised a special form of bridge which after this time 

 was employed for the purpose for many years. 



The Electrical Standards Committee was reappointed at the Swansea 

 meeting in 1880, and in the following year 1 881, at York, one of the present 

 authors (R. T. G.) became connected with the work ; he was formally 

 appointed Secretary at the Southport meeting in 1883, and from that date 

 up to the year 1919 the coils were in his charge. They are still at the 

 National Physical Laboratory. The Committee was dissolved in 191 2, 

 when its Reports were collected in a volume published by the Association 

 under the editorship of Mr. F. E. Smith (now Sir Frank Smith, Secretary, 

 Royal Society). Up to that date comparisons of the coils aniong themselves 

 were continually in progress. Between 1881 and 1884 their values were 

 determined in ohms and also in terms of the length of a column of mercury 

 by Lord Rayleigh and Mr. Glazebrook. In 1888 a further very detailed 

 examination of their values was made by Mr. Glazebrook and Mr. Fitz- 

 patrick. In 1908 Mr. F. E. Smith reported very fully on their values and 

 on the changes which had occurred ; while the concluding portion of this 

 paper consists of an account of a comparison made during the current year 

 by Dr. Hartshorn at the National Physical Laboratory and a discussion of 

 the results up to date. 



The records show that most of the coils have changed appreciably during 

 their long life, but that the two platinum coils marked 35 and 36 ^ in the 

 original table have remained unchanged. 



Table U. 



Resistance of Mercury Column 

 100 cm. long and i sq. mm. section at 0° c. 

 IN B.A. Units. 



Value found in 1881 by Lord Rayleigh, 



corrected for temperature of cups . = 0*953 88 B.A.U. 

 Valuefoundini888byR. T. G. . = 0-953 52 B.A.U. 



Resistance of i B.A. Unit 

 in terms of Length of Mercury Column. 



Value found in 1881 by Lord Ray- 

 leigh ...... = 104-842 cm. 



Value found in 1888 by R. T. G. • = 104-875 cm. 



The relative changes are known during the period in question, and 

 Table II, based on Sir Frank Smith's Report of 1908, is of importance 

 as showing that during the period 1880 to 1888 the value of the B.A. 

 unit expressed in terms of mercury remained unaltered. The diagram and 

 tables given later in the Report enable the changes which have taken place 

 to be followed in detail. 



* About 1880 the coils were re-marked, and these coils have since been known 

 as D and E. 



