424 
LORD RAYLEIGH AND MRS. H. SIDGWICK OH THE 
The resistance in tbe other branch was 25’95648, so that the ratio of galvanometer 
constants is determined to be 
25’95648/9’97890 = 2’60113. 
It will be seen that even with a single cell the sensitiveness was such that the errors 
of reading could scarcely exceed TblVool indeed, the weakest part of the arrangement 
is in the standard resistances. 
With use of the above resistance coils the values obtained for coil B on three 
occasions were 
2’60087, 2-60098, 2-60113, mean 2-60099. 
As a further check, the experiment was repeated with a different combination of 
resistance coils. The 26 was replaced by 13. made up of three singles and of the 
same [10], while the [10] was replaced by a [5]. Two experiments gave 
2-60046, 2-60026, mean 2-60036. 
The mean result of the two arrangements is thus 2"60067. The difference is about 
Tob'oj and would be explained by an error of -§ 0 \> 0 - in the value of the [1 ()].'" 
For coil A of the dynamometer the ratio of galvanometer constants was found in 
like manner to be 2‘60072, the close agreement of which -with 2 - 60067 is a verification 
of the winding and insulation of the coils. For the further calculations we require 
only the mean, and we therefore take as the ratio of galvanometer constants for the 
ebonite coil and a coil of the dynamometer 
2-60070. 
The accuracy obtained in the above determinations is doubtless quite sufficient for tbe 
purposes of the present investigation, but if it were desired to push the power of the 
method to its limit it would be necessary to design tbe coils so that tbe ratio should 
be (approximately) expressible by very simple numbers. If in the present case, for 
example, we were content to sacrifice one-fifth of the number of turns on the ebonite 
coil, the ratio could be made to approach that of 2 : 1. Tbe standard resistances might 
then be composed of three equal resistance coils, which could be more accurately 
combined and tested than the more complicated combinations that we were obliged to 
use. In such a case the limit of accuracy could probably depend upon the difficulty 
of adjusting the coils under comparison and the suspended magnet to their proper 
places. It is scarcely necessary to say that care must be exercised in the disposition 
of tbe leading wires, and that the direct action of the current in the principal coils 
upon the needle of the bridge galvanometer must be tested, and, if necessary, allowed 
for. 
* Eor the methods used to find the values of the [24], Ac., reference must be made to former papers. 
