ON PRACTICAL STANDARDS FOR . ELECTRICAL MEASUREMENTS. 41 



From Tables V. and VI. it appears to be practically certain that tlio 

 coils B, D, E, and H have the same resistance in 1908 as they had in 

 1888. The agreement of the values for D and E is very remarkable, for 

 the temperatures at which these coils were believed to be correct in 1888 

 are stated to the nearest tenth of a degree only ; an apparent change in 

 resistance of 15 parts in 100,000 would, therefore, have been negligible. 

 With respect to G, it has risen by over 1 part in 1,000 during the past 

 5 years and Flat changed by 17 parts iii 100,000 in 1902.^ The fluctu- 

 ations in the value of H are believed to have amounted to about 1 part in 

 10,000 during the period 1888-1908.^ 



Of the four coils B, D, E, H, apparently constant for the period 1888- 

 1908, we have already concluded fi-om the diflerences given in Table IV. 

 that B, D, and E have remained approximately constant since 18G7. 

 One of the coils D-E, appears, from Table IV., to have changed in the 

 interval 1867-1876, and the apparent change corresponds with the change 

 resulting when one of the coils is lowered 0°*2 C. in temperature. It is, 

 however, practically certain that the change is only apparent. The tem- 

 peratures at which the platinum coils were stated to be correct in 1865, 

 1866, and 1867 are given by Mr. Hockin in the Report for 1867. They 

 are as follows : 



1 B.AU. at 15°-7 C. January 7,1863 

 Coil No, 35 (D). ■{ 1 „ „ ]5°-7C. August 18,18(515 



1 „ „ 15° 7 C. February 10, 1867 



15°-5 C. January 7, 1863 



Coil No. 36 (£),<{ 1 „ „ 15°'5 C. August 18, 1866 



15°-7 C. February 10, 1867 



'■{; 



In the Report for 1888 the temperature coefficient of D is given us 

 0-00308 B.A. Unit, and of E as 000302 B.A. Unit. These values agree 

 closely with those given in Table III., and they have been used in the 

 compilation of the following complete list of the difference values (D-E) 

 which now deserves attention : 



D-E = -59 X 10-' B.A.U. at 16°0C. Year 1865 



„ = —59 „ „ ,, „ 1860 



,, = 1 ,) .1 .1 i< 1867 



„ = -G3 ,, „ „ „ 1876 



= -50 ., „ „ „ 1879-81 



„ = —60 „ ,. „ „ 1888 



= -60 „ „ ,. „ 1908 



The conclusion is obvious. The original difference between the coils 

 was approximately 60x10"^ B.A. unit and has remained constant ever 

 since. There is little doubt that the difference recorded for 1867 is 

 incorrect ; it may easily happen that there is a difference of 0°'2 C. 

 between the apparent and true temperatures of a coil embedded in paraffin 

 wax, and such a difference would completely explain the 1867 result. 



This conclusion necessitates a revision of the difference values in 

 Table IV. The corrections are easily made, for the differences A-E, 

 B-E, C-E and E-CI should be respectively equal to the differences A-I>, 

 B-D, &c. 



We believe that the two platinum coils have remained constant in 

 resistance since 1867, and that the values in 1867, 1879-81, 1888, and 

 1908 of these and other coils in terms of the original B.A. unit (1867) 

 ftre Q,s follows : 



I firit, Assoc. Sep., 1903, . - ? Thid. 



