VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
249 
We shall retain the whole series of observations and take as the resistance of our 
standard coil R at a temperature of 14°'6 C., the value 
158-626 eaIth qKad r nt3 
second 
It remains now to explain the method used to determine the values of the 
resistances. 
For this purpose the coils in a post office resistance box, made by Messrs. Elliott 
Brothers, were compared with the standards at the Cavendish Laboratory. The 
1 unit coil of the box was compared with the coil known as Flat in Professor 
Chrystal’s report, then the 1-unit -j- Flat were balanced against the 2-unit coil of the 
box, then this 2-unit against the second 2-unit, which we will denote by 2', then 
1 + 2+2' against the 5-unit coil, and so on. 
In this manner all the coils between 1 and 2000 B.A. units were compared. 
The British Association wire bridge was used in making the comparison. 
In the ordinary use of this (Carey-Foster’s method) the two coils to be compared 
are connected to the ends of the bridge wire and a measurement taken, the coils are 
then interchanged and another observation is taken, and from these two the difference 
between the coils is expressed directly as the resistance of a portion of the bridge wire. 
We, however, could not apply this method, for, calling P and Q the coils to be com¬ 
pared, since P and Q are coils in the same box, one end of P is always in electrical 
connexion with one end of Q. The following arrangement therefore was adopted :— 
Two coils of known resistance were connected one to each end of the bridge wire, 
while P and Q formed the other arms of the bridge. The coils actually used were 
those marked F and G in Chrystal’s report. 
The sliding contact was adjusted till no current passed through the galvanometer, 
and its position noted. 
Let a-j-x be the resistance of the portion of the wire connected with F, a—x of 
that connected with G, so that 2a is the whole resistance of the bridge wire. Let 
1 + SF, 1 + SG be the resistances of F and G at the temperature of the observation. 
SF and SG are very small. 
At 14° 
SF=—’00084 ohm 
SG=- ’00112 „ 
Then we have 
P_1 + SF + « 4 x 
Q 1 4 SG 4a —x 
Interchange F and G and let x be the new value of x 
P 1 4 SG 4 a 4 x' 
Q 1 4 SF 4 a — x' 
Whence cc+a/=SG —SF. 
MDCCCLXXX1II. 2 K 
