AND STRAIN ON THE ACTION OP PHYSICAL FORCES. 
43 
For the purpose of this particular part of the enquiry, arrangements were made as 
in fig. 7, where it will be seen that the crossed pieces a a, b b, which are insulated from 
each other, serve to connect the tenths and units with the resistance-coils a. 
Examination of the resistance-coils and of the platinum-iridium wire. 
In order to test the uniformity of the platinum-iridium wire N N, two platinum- 
silver wires, of resistances 9 and 10 ohms respectively, were placed in the positions of 
0 and S, fig. 7. 
was supplemented by a rheochord, and at botli a and /3 the plugs were inserted in 
the 10 ohms resistance, whilst the rheochord was employed to bring H to the extreme 
right-hand end of the graduated portion of the wire. If, then, r denote the resistance 
of the ungraduated portion of N N on the right of H, together with the interposed 
portion of the rheochord and Y, whilst l denotes the resistance of the ungraduated 
part of N N on the left of H together with X, we have 
or 
10 + Z + (47Q-w 1 )a! 
10 + ? ' + n v x 
resistance of 0 
resistance of S 
— h say, 
10 -f- / -j- (4/0 — n-^jx —&( 10 —J— 7' —|— iq.cc) 
(1) 
where x is the average resistance of one division of N N, and n l is the number of 
divisions from the right-hand end of the graduated wire at which H is placed. 
A resistance of ygth ohm is now added to a from the tenths’ dial-plate, and in 
consequence H has to be moved to n. 2 divisions from the right-hand end in order to 
restore the balance ; 
l0-l + l+(A70—n 2 )x=lc(l0 + r+n 2 .x) .(2) 
from (1) and (2) we obtain 
_ -1 
X (k + l)(?t 3 —7q)‘ 
Again the rheochord is adjusted so as to bring H about 10 millims. still further away 
from the right to, say, divisions, and on the removal of the x^th ohm resistance H is 
brought back towards the right to, say, n 5 divisions ; then as before 
__ -1 
X ~(k+1)(n i -n s )' 
If the wire is uniform the value of x in the latter case should be equal to that of x 
in the former, and therefore 
a 3 — n. 2 fq 
G 2 
