VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
261 
Two arms were the two standards I wished to compare; the coil It of about 
160 units and Lord Rayleigh’s standard, which we will call X, of about 24. 
The third arm was two single unit coils and a 5-unit arranged in series, and the 
fourth arm a single unit, the coil Flat. 
The two single units were the B.A. units C and G of Professor Chrystal’s report, 
their values and that of the 5-unit are accurately known in terms of the mean B.A. 
unit. v 
In all cases the electrodes of the coils were well amalgamated and rested securely 
on the copper discs at the bottoms of mercury cups. 
The electrodes of the Elliott box were connected with the two ends of the 7 -unit 
arm, so that the box formed a shunt, and by altering the plugs in the box the 
effective resistance of the arm could be finely adjusted, and the ratio of the resistance 
of the Flat coil to that of this arm made equal to that of It to X. 
Thus I found that with 164 units out of the box there was a deflection of the 
galvanometer of +33-2 scale divisions, while with 163 out the deflection was —5 '6. 
Thus the true value of the shunt is 163-143. Correcting this to the proper tempera¬ 
ture we find that the effective resistance of this third arm is 6'70438 units, that of the 
Flat coil at the same temperature being ‘99944 unit. 
Xow the temperature of R at the time of observation was 13°"3, that of X being 
13°-4. 
Hence 
hl3 .3 _ 6 7 0 4 3 8 
v — 99944 
Whence substituting the value of X and reducing to the standard temperature 12° 
R 0 —160\523 B.A. units. 
A second experiment was made at a different temperature, and instead of noting 
the deflections of the galvanometer produced by altering the box by 1 unit a second 
shunt was introduced and varied until the deflection was zero: the value of this shunt 
was 30,000 units. 
From this experiment I found 
R u =160 , 518 B.A. units. 
We take as the true value of R () deduced from .these two experiments 
R 0 = 160-520 B.A. units. 
We have now to compare directly the values of S 0 and R 0 . S 0 is about 3060 units, 
so that the ratio of S 0 to R 0 is between 19 to 1 and 20 to 1. 
Four sets of coils were, therefore, arranged for a Wheatstone’s bridge, two arms of 
which were S and R, the other two arms being 20 units—made up of the two 5 and 
10-unit standards already described, while the fourth was the single coil G. 
