VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
303 
firmation, inasmuch as leakage of electricity from the main circuit into the galvano¬ 
meter branch would exert a different influence in the two cases. The observations 
were not reduced until some time afterwards, and it then appeared that the agreement 
was even better than it would have been reasonable to expect. 
§15. The final number, ’9867 X 10 9 , expressing the value of the B.A. unit in 
absolute measure as determined by these two series of observations, is almost identical 
with that previously obtained by ourselves, and by Glazebrook, using other methods. 
With respect to the independence of these determinations, the only thing calling for 
notice is the fact that the same induction coils were employed both by Glazebrook 
and in the present investigation. In other respects there has been, we believe, 
scarcely any point of contact. But it is evident that an error in the measurements 
of mean radius of these coils must propagate itself into both results. The point to 
which we now wish to direct attention, is that the error of mean radius will influence 
the final number in opposite directions. In the method employed by Glazebrook, an 
under-estimate of the mean radius would lead to an uncler-estimate of the induction 
coefficient, whereas with us it would lead to an over-estimate of that quantity. So far, 
therefore, as the error of mean radius is concerned, it would appear that the use of the 
same coils is far from impairing the value of the results. Even with respect to the 
number of turns, an error, if that be supposed possible, would affect the results in a 
different manner, for Glazebrook was concerned with the product of the numbers for 
the two coils, while we evidently are concerned with the sum. 
§ 16. In researches of this kind it is proper to calculate the in flue nee upon the 
result of errors in the fundamental measurements. The value of M depends upon 
three linear quantities : the radius of the disc (a), the mean radius of the two coils (A), 
and the distance between their mean planes (2b). In the present case, however, the 
latter element enters in a very subordinate degree. From § 25 it appears that 
M A a 
It has been shown" that these conditions compare favourably with those of most of 
the other methods that have been employed. From its nature a is much more easily 
measured than the diameter of a coil. 
§ 17. The results deduced from the several days observations, when corrected for 
slight variations of temperature of the resistance coils, &c., exhibit a remarkable 
accordance. By reference to the tables (§ 27) the reader will see that the maximum 
divergence from the mean in Series I. is only about one part in 4000, while in Series II. 
it is even less. We were thus encouraged to carry out a modification of the method 
which we had had in view all along, and the results of which would be in great 
measure independent of those of Series I. and II. 
* Phil. Mag., Nov., 1882. 
