VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
311 
The [5js were determined in two ways. Five singles were combined in series and 
compared with one of the [5]’s ; afterwards the two [5]’s were compared with one 
another. In the second method, which is probably preferable, the sum of the two 
[5]’s was found by comparison with the [10 j. From the sum and difference the 
separate values can of course be deduced. 
The measurement of the [+□] demanded some precaution on account of its smallness. 
Two standard singles, the [10], and the +j, were combined with four insulated mercury 
cups, and without the use of connecting pieces, so as to form a Wheatstoxe’s balance 
(fig. 5), care being taken to bring the associated battery and galvanometer terminals 
into immediate contact with the legs of the [j^] (see § 12). To get the means of 
adjustment, a box, giving resistances up to 10,000, was placed in multiple arc with one 
of the singles. If, as was the case, the four coils be so nearly in proportion that a 
resistance of several hundreds from the box is needed for balance, the delicacy of the 
arrangement is all that can be desired. Headings are taken also with battery reversed, 
to eliminate thermo-electric disturbances. Especial pains were taken with the measure¬ 
ment of the [Jo], and of the [10], errors of which would be propagated into the results 
of all three series. 
§ 27. The various temperatures of the coils at the time of use, and the fluctuations 
from day to day, complicate the calculation of the effective resistances R 1 and IF, 
which in principle is simple enough. The results are given in column II. of the 
Tables. Thus in the first series on July 14, when the effective resistance was ’004407G 
B.A., as calculated from the values of a, b, c, for the observed temperatures of the 
coils, the effect (E-W) of reversing the battery key (corrected for direct effect) was 
— 30 divisions of the galvanometer scale, the direction of rotation being positive. When 
the effective resistance was altered to ’0044430, the difference E-W became +10 
divisions. From these results we infer that E-W would vanish for the effective 
resistance ’0044341, as given in column Y. The corresponding result with negative 
rotation is given in column YI. These resistances relate to the actual speed of 
rotation determined by the frequency of vibration of the electric fork (§ 6). To 
render the results of different days fairly comparable, two small corrections have to be 
