AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
99 
Experiment XXXI. 
A wire of annealed iron, 8 feet in length and '092 centim. in diameter, stretched 
by hand. 
Total increase of length 
per cent, produced by 
stretching = A. 
Total increase of specific 
resistance per cent, 
produced by stretching 
= B. 
Ratio of 
B : A. 
5’74 
-T05 
-•0183 
7-51 
-•143 
-•0190 
9-49 
-■156 
-•0164 
11-56 
-•177 
-•0153 
16-36 
-•150 
-•0091 
Experiment XXXII. 
A wire of annealed nickel, 8 feet in length and '106 centim. in diameter, stretched 
by loading. 
Load in 
kilogs. 
Total increase of length 
per cent, produced by 
the load = A. 
Total increase .of specific 
resistance per cent, 
produced by the load 
= B. 
Ratio of 
B: A. 
12 
0-32 
-0-757 
-2-366 
18 
1-67 
+ 1-260 
0-755 
20 
3-06 
+ 1-5.14 
0-494 
22 
4-82 
+ 2-052 
0-426 
24| 
7T4 
+ 3-634 
0-509 
It will be observed that of the four metals—silver, copper, iron and nickel—the 
two former are at first increased in specific resistance, and that the increase only 
continues up to a certain amount of stretching when, having reached a maximum, 
it begins to decrease; whilst with the two latter the first alteration of resistance is 
in the way of a decrease, which also after attaining a maximum begins to diminish, 
and with nickel there is soon a comparatively large increase. Of nine metals which 
have been examined, iron and nickel are the only two which in the first instance 
show a decrease of specific resistance, whilst the remaining seven are similar to copper 
and silver in respect to the change of specific resistance due to permanent extension. 
In the following table the effects of different amounts of permanent longitudinal 
extension on the specific resistance of copper, zinc, German-silver, nickel, platinum, 
iron, aluminium and silver are shown by a series of curves. The abscissse of these 
curves represent the percentage alteration of specific resistance, and the ordinates the 
percentage alteration of length, the former on a scale of '0125 percentage alteration to 
the millimetre, and the latter on a scale of '25 percentage alteration to the millimetre; 
o 2 
