42 



EEPORTS ON THE STATE OF SCIENCE. 



Table VII. 



Resistances at 16°'0 C. in terms of the original B.A. Unit (1867). 

 ( Values obtained through the two Platinum Coils D, E.) 



From Tables IV. and VII. it is clear that the maximum number of 

 coils which can have kept constant is two, and if the platinum coils have 

 not remained constant then one only of the other coils can have done so. 

 Since D and E are of pure platinum, and not of an alloy, it is probable 

 that these would change least. 



If our conclusions are correct, the results are not only of some value 

 as showing the changes which may take place in the resistance of certain 

 alloys when embedded in paraffin wax, but they are also of value because 

 the coils link together so many determinations of the ohm in absolute 

 measure and of the specific resistance of mercury. It is not convenient 

 1o collect the various determinations here, but as an instance of the 

 uses to which the data given in this Appendix might be put we take 

 Lord Rayleigh's and Mrs. Sidgwick's determination in 1881 ^ of the 

 specific resistance of mercury. It was found that 0-95412 B.A. unit was 

 equal in resistance to a column of mercury 100 cm. long, 1 sq. mm. in 

 section, at 0° C. Now in Lord Rayleigh's experiments the terminals of 

 the mercury standards were not at 0° C, but at 5° or 6° C, and it was 

 shown by Dr. Glazebrook ^ in 1888 that an error of about 0-00024 was 

 almost certainly introduced because of this. If we apply a correction of 

 this amount, Lord Rayleigh's value becomes 0-95388 B.A. unit as the 

 resistance of 100 cm. of mercury at 0° C. The coils F and Flat were used 

 in the 1881 determination, and the values of these coils were taken from 

 Fleming's chart. They were therefore : 



r = 0-99971 B.A. unit at lG°-0 C. 

 Flat = 1-00034 B.A. unit at 16°-0 C. 



(From Table II.) 

 (From Table II.) 



From Lord Rayleigh's observations, therefore, 



F at 16°0 C. = 0-99971/O-95388 = 104-805 cm. mercury ; and 

 Flat at 16°0 C. = 1-00034/0-95388 = 104-871 cm. mercury. 



At the present time (1908) 



F at I6°-0 C. = 104-959 cm. mercury ; and 

 Flat at 16°0 C. = 104922 cm. mercury. 



Using the 1908 values and the changes in F and Flat, recorded in 

 Table VII., we conclude that in 1881 



F at 16°0 C. was equivalent to 104808 cm. mercury; and 

 Flat at 16°-0 C. was equivalent to 104874 cm. mercury. 



' Phil. TruM., vol. 174, p. 173. 

 « Ibid., A, 1888, pp. 375-6. 



