STANDARDS OF RESISTANCE, 1865-1932 



427 



Two other interesting tables of values recording the history of these coils 

 were given by Mr. F. E. Smith in 1908, and these have also been brought 

 up to date. Table VII records the values of the coils in terms of the B.A. 

 unit in general use during the period 1891-1903, and Table VIII gives the 

 values obtained on a number of occasions in centimetres of mercury. 



Table VII. 



Values of Coils at i6-o° C. in B.A. Units in 1888, 1908 and 1932, 



obtained from Comparison with Mercury Tubes, assuming the Resistance 



of I metre of Mercury to be o • 953 52 B.A.U. 



* Values subject to a probable error of 4 parts in 100,000, due to the fact that the 

 terminals of the mercury tubes were not exactly at 0° C. No correction has been made 

 for this, since the probable error of the observations was of this order. 



t Values for a current of o ■ 12 ampere (see Table V). 



(iii) The Value of the B.A. Unit. 



When changes in the relative values of the coils were first noted, the 

 B.A. unit was taken, as already stated, to be the mean value of the six coils 

 A, B, C, D, E and G at the temperatures at which they were originally stated 

 to be equal. This has been referred to as the mean B.A. unit for the year 

 in question. The data now available show that the changes in the unit so 

 defined have been approximately as given in Table IX (p. 429). 



The value generally used for the ratio of the international ohm to the 

 B.A. unit is — 



I international ohm = i -013 58 B.A. unit (1892), 



this being the value accepted in 1892. However, as the coils were changing 

 in a manner not accurately known, it is evident that the unit in practical use 

 at the Laboratory, normally the mean of all the coils, was a variable quantity. 

 For example, in 1903 Mr. F. E. Smith made an estimation of this ratio, 

 taking the platinum-silver coils as standards of reference (these are the ones 



