AND STRAIN ON THE ACTION OP PHYSICAL FORCES. 
51 
unit load increases less and less rapidly up to a certain degree of stress, and after¬ 
wards begins to increase more and more rapidly, whereas with the annealed metals 
the average alteration increases at first by almost equal amounts. 
Experiment VIII. 
A piece of platinum (1) annealed was loaded and unloaded several times with 
12 kilogs., and was immediately afterwards tested, with the following results : — 
No. of kilogs. in 
load. 
Temporary alteration 
of resistance. 
Average alteration 
per kilog. 
Difference between 
consecutive averages. 
2 
66'5 
66-5 
4 
134-0 
67-0 
•5 
8 
276-0 
69-0 
2-0 
After a rest of 20 hours with all stress removed except that of scale-pau. 
2 
67-0 
67-0 
4 
136-0 
68-0 
1-0 
8 
281-0 
70-25 
2-25 
From the last experiment it is evident that part of the increase of elasticity which 
is gained by repeated heavy loading and unloading is lost by allowing the wire to 
rest, and, moreover, that the departure from “ Hooke’s law ” which we have seen to 
be appreciably lessened under the influence of the former cause is increased again by 
the latter. 
In Table I. will be found the values of the specific resistance of the different metals, 
the increase of resistance per unit which is temporarily produced by a stress of 1 grin, 
per square centimetre, the increase of resistance per unit which would be caused by 
stress sufficing to double the length of the wire, and the increase of specific resistance 
per unit which would be caused by stress sufficing to double the length of the wire ; 
the specific gravity and section of the wire are approximately the same as those 
recorded in Part I. 
H 2 
