VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
301 
improvement. The increase of galvanometer deflection was not really of much advan¬ 
tage, since the difficulty of getting sharp results arose from electromotive disturbances, 
and these were magnified in the same proportion. It would probably have been 
better to have retained the unit in b, and to have replaced the galvanometer by one 
of higher resistance. 
§ 11. Preliminary trials having given apparently satisfactory results, we proceeded 
to make regular series of observations in the manner already described. We had not 
gone far before anomalies revealed themselves of such a character as to prove that we 
were not yet masters of the method. It usually happened that each day’s observa¬ 
tions agreed well together, showing that the sensitiveness was sufficient; but when 
we came to compare the results obtained on different days unaccountable discrepancies 
became apparent. The first result of the more severe criticism to which the arrange¬ 
ments were then subjected was to show that sufficient thought had not been given 
to the question of insulation. The wire composing the induction coils, or rather one 
extremity of it, is necessarily at a high potential, and a very moderate leakage from 
the coils to the frame, and thence to the disc, might cause great disturbance. Some 
such leakage was in fact detected on application of appropriate tests. Ebonite insula¬ 
tion was accordingly introduced into the supports of the coils. The battery was care¬ 
fully insulated from the ground, as was also the frame carrying the revolving disc, and 
other precautions were taken which it is unnecessary here to detail. For the sake 
of definiteness one point of the galvanometer commutator was connected to earth. 
With these improvements tests were satisfied more severe than that of actual use, and 
these tests were renewed at intervals during the spinnings. 
The results however still showed that some defect existed which we had not yet 
succeeded in detecting. It made no appreciable difference which way the disc rotated, 
but the means of different days’ work failed to exhibit the desired accordance. Two 
months’ work had already been spent upon the experiments, and we had begun to 
despair of a satisfactory issue, when it occurred to us that the connexion of the coils 
for compounding the effective resistance was faulty. 
§ 12. By reference to fig. 1 it will be seen that the main current traverses part of 
the cup C, and that part of the same cup is also included in b. Now, although for all 
ordinary purposes the resistance of the parts of the cup might be neglected, in the 
present case it is the small effective resistance It with which it comes into comparison. 
If we aim at an accuracy of TooWo> we cannot afford to overlook a resistance entering 
1 this manner, even though it may not exceed tooAocTo ohm. The discrepancies were 
doubtless due to small differences in the position of the wires and coils in cup G, 
moved as they were from day to day in order to verify the soundness of the contacts. 
In order to avoid the difficulty we have only to take care that no part of b can 
possibly be traversed by the main current, and this is easily done by the introduction 
of another mercury cup. Fig. 3 shows the arrangement adopted. The main current 
enters at the cups A and D, and the greater part is taken by the two unit coils in 
