ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



39 



place, and the task before us is to select the most constant and the most 

 variable coils. In all such cases a table of difference values is most 

 helpful. Table IV. gives such values for the six coils A, B, C, D, E, G 

 in 1 X 10--' B.A. units at 16°0 C. 



We conclude from the differences given in column 7 and the tempera- 

 tures given in the last column of Table IV. that B and E have possibly 

 remained constant during the period 1867-1908 and that C and D are 

 ne.xt in order of constancy. The coils D and E have remained relatively 

 constant since 1876. 



Dr. Glazebrook in 1888 measured the B.A. unit in terms of the 

 specific resistance of mercury, and found that the value of the resistance 

 of a column of mercury, 1 metre long, 1 sq. mm. in section, at 0° C. was 



0-95352 B.A. unit. 

 For the purposes of the comparison. Dr. Glazebi-ook used the two coils F 

 and G, and their values are given by him as 



F =0-99807 B.A.U. at 10' C. 



G=0-99778 „ „ 10° C. 



These values were taken from Fleming's chart, and when corrected to 

 16° C. they are practically identical with those recorded in Table II., as 

 they should be. Flat was also used (0-99857 B.A.U, at 10° C), but 

 observations during the two years preceding 1888 showed that it was 



Table IV. 



Difference Values mix 10"^ B.A. Units. 



* Hockin (1867 Report) gives the temperatures at which D and E were correct 

 in 1865, 1866, and 1867. From the values given by him it appears that the difference 

 D-E was -59 X 10"* B.A.U. at 16°-0 C. in 1865, -59 x lO"' in 1866, and 1 in 1867. 

 These diflferences, taken in conjunction with those given in the above table, make 

 it practically certain that the difference given for 1867 is incorrect, and should be 

 replaced by a difference of the order -60. 



