AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
47 
the number of millimetre divisions of this wire which have a resistance equal to the 
branches a and fi respectively, including in these the connecting wires X and Y and the 
parts of the divided wire on either side of H, n he the number of divisions through 
which it is necessary to move H in order to restore the balance of resistance when 
disturbed by a load W, and S be the section of the wire, then, denoting by x the 
increase per unit of resistance which would be produced by a unit load acting on unit- 
area, we have within a sufficiently close degree of approximation 
(A + B) xnx s 
X ~ XxBxW * 
Again, if x be multiplied by e we obtain the alteration of resistance per unit which 
would result from doubling the length of the wire by the application of longitudinal 
traction ; therefore denoting this latter value by y, we have 
y—exx. 
Part of y is due to mere increase of length and diminution of - section; this part 
= l + 2<x very nearly. Thus the alteration of the specific resistance produced by the 
traction 
= y -( 1+2°-). 
It will be seen that x and y are calculated on the assumptions that the change of 
resistance is directly proportional to the stress and also to the elongation ; both these 
assumptions were found to be nearly correct, but neither are strictly so. 
The following experiment out of many will serve to show («) that the temporary 
alteration of resistance is nearly but not quite proportional to the load, and ( b ) that 
it is possible to measure with considerable accuracy minute changes of electrical 
conductivity even when the resistance of the wire used is small. 
Experiment I. 
An annealed copper wire T54 centim. in diameter and having a resistance of 
only ‘0224 ohm was loaded by pouring 12 measures of water, each having a weight of 
5825 grms., into a pail attached to the pulley on the wire. This load, which 
was four-fifths of the “ breaking-weight,” was suffered to remain on the wire for 
some hours and was then removed. The following consecutive observations were 
begun next day and extended over three days, two trials being made on each. 
The numbers in the column headed “ Temporary alteration of resistance ” are the 
divisions of the platinum-silver wire N N, through which it was necessary to shift H 
in order to balance the effect of the load. 
