ON THE SPECIFIC RESISTANCE OF METALLIC WIRES. 371 
These instruments were arranged, as shown, on a table T ; the wires a and 6 
were carried along side by side above the table ; the former passed over a pulley 
and was attached to a scale pan, on which the stretching weights were placed. 
In fitting these arrangements, the resistances of the coils CC were first 
carefully measured and made as nearly equal as possible. These coils were 
then fixed in their positions, and the middle point of the coils and bridge wire 
determined as follows :—two equal resistances R and R, were introduced into 
the bridge, and the position of the key K found, where no deflection was ob- 
tained when it was depressed; R and R, were then interchanged and the 
position of K for zero deflection again determined. The resistances RR, were 
then varied, until, by interchanging, the same position was obtained for K ; this 
point was taken as the middle of the coils and bridge wire. 
The value of one division of the scale S was then determined by varying 
one of the resistances RR, by a known amount, and noting the number of 
divisions the key K had to be moved in order to again establish a balance. 
If this number of divisions be 7, the total reduced length of the bridge wire 21, 
and the resistances introduced be R and R+7, we have the following equation 
for the value of / in scale divisions. 
Pea Ee 
l4+n R+7r; 
and therefore 
/= — 
This supplies the data necessary for the determination of a and 0 at any time. 
Suppose now that at the beginning of an experiment 
aoe A 
6.7 BZ 
and that after the wire a had been stretched 
a,_ Ay 
joy ye 
Then we have 
aa | oe yh: 
ater and therefore ao x 
Again, suppose that at the beginning of the experiment the length of a is J, 
and after stretching 7. Then the ratio of the resistances before and after 
stretching is —_ (according to the definition of specific resistance given 
above), or the supposition that the specific resistance does not change. The 
percentage increase of resistance can be obtained from these two equations ; 
it is equal to 
) A, Bi? 
