VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
241 
Thus 
R+x'p_ IT 
ll + y'p~~W 
But 
x+y= x '+iy' 
Hence 
v R +x p =It +yp 
~R=ii+(x'—y)p 
A number of experiments were made to determine p, and the value p= '000072 B. A. 
unit was obtained. 
Since p only comes in as a small correction, we may take one B. A. unit as 1 ohm. 
Again, the value of R depends on the temperature, and our experiments required 
reducing to a constant temperature t 0 ; let t be the temperature of R at the time of 
experiment, R 0 the value of R at temperature t 0 , and a the coefficient of increase of 
resistance per degree centigrade. Then we have 
R=R 0 { ld-a(f — £ 0 )} 
Hence, finally our equation (4) becomes 
T, (, , /. , 27TM S + V ’2(q 1 + qc) \ 1 lp 2 — 8^ 2 . . . 
R 0 {l + a(«-y}=— x\‘X —X 1+ 3 9 „ 8 \ + (x-y)p . . (o) 
T l + o 
(.lh +^ 2 ) 
3 2 « 2 
The experiments were made in the following order :— 
The time of swing was observed, the secondary circuit being closed as in the 
experiments. 
The variable resistance in the secondary circuit was adjusted until the difference 
between R and R could be measured in terms of the bridge-wire resistance, and the 
values of x y, x y' determined. While this was being done a second observer read 
the temperatures of the coils R, S, and Y, and the galvanometer G. 
The connexion P P', Q Q' were broken, and P Q was joined. The resting point of 
the reflected image of the scale was observed when no current was passing through 
the galvanometer. This was done in the usual manner by observing five consecutive 
turning points. 
The galvanometer needle was brought as nearly as possible to rest by the use of a 
damper. This consisted of a coil of wire placed near the needle, through which the 
current from a single Leclanche cell could be passed. By means of a second key a 
shunt could be introduced into this circuit so as to allow only a small fraction of the 
current from the battery to circulate in the coil. After a little practice the apparent 
MDCCCLXXXIil. 2 I 
