AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
13 
Experiment IX. 
Number of kilogs. in load. 
Average alteration per kilog. 
Millimetre. 
2 
•830 
4 
•835 
6 
•840 
8 
•847 
10 
•866 
12 
•911 
It is here seen that the alteration per kilogramme rapidly increases when the loads 
become heavy, and yet in both these last experiments the wire recovered its original 
length on the removal of the load. It is evident, therefore, that in the case of 
annealed copper the length increases in greater proportion than the load, and this 
was proved to be the case whether the wire was allowed to rest loaded or unloaded. 
Similar results were obtained with annealed platinum, silver, aluminium, platinum- 
silver, German-silver, and zinc. With soft iron, however, the -case is different if the 
wire be weighted for some time after permanent extension has taken place. An 
examination of Experiment XIII. shows that under these circumstances the average 
alteration decreases up to a certain point as the load increases. 
It remains now to consider the case of iron allowed to rest unloaded. 
Experiment X. 
An annealed iron wire, after having been permanently elongated by traction about 
8 per cent., remained unloaded for several days, and was afterwards tested with weights 
up to 10 kilogs. 
Number of kilogs. in load. 
Alteration of length per kilog. 
1 
Millimetres. 
1-350 
2 
1-355 
3 
1-367 
4 
1-376 
5 
1-386 
6 
1-388 
7 
1-386 
8 
1-389 
9 
1-392 
10 
1-394 
Here the average alterations of length increase with the load, though not to the 
same extent as with copper. 
In both the last experiments with iron the recovery of the wire after the removal 
of the stress was so perfect that the zero position of the vernier was not shifted one- 
tenth of a millimetre. 
