AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
15 
same effect as continued rest, a fresh pair of iron wires were suspended and tested in 
the usual manner with the scale and vernier ; but it was ascertained that vibrating a 
wire under these circumstances did not produce any immediate appreciable effect on 
the elasticity. It would appear, therefore, that the effect of rest is not felt when 
the temporary elongations are very small. 
Permanent Alteration oe Elasticity Produced by Permanent Extension. 
Experiment XI. 
The same wire as in Experiment X. was further lengthened by 7, 15, 10 and 
7^ centims. respectively on four separate occasions, and after each elongation the load 
was removed, and a rest of 24 hours allowed. After each rest the temporary alteration 
of length produced by 8 kilogs. was determined, the vernier after each permanent 
extension having been shifted to its original position. 
Actual alteration 
observed. 
Calculated alteration 
which would be produced 
on wires of the same 
section as in Experiment V. 
Total percentage of 
permanent extension 
produced before testing. 
12-88 
12-88 
4-0 
13-02 
12-87 
51 
13-32 
12-87 
7-5 
13-50 
12-84 
9-0 
13-65 
12-83 
10-2 
These results are the means of several observations in each case, and show that the 
elasticity of copper is very slightly increased by these particular amounts of permanent 
extension, when the wire is allowed to rest unloaded. 
A similar effect was proved to be produced on copper wire which was kept loaded 
for some time after permanent extension. 
Experiment XII. 
A piece of annealed iron wire, 860 centims. in length, was elongated by traction 
to the extent of 21 - 7 centims. so as to make it perfectly straight, and afterwards 
allowed to rest unloaded for two days ; a set of experiments was then made which 
resulted in giving a mean value of 8'28 half-millims. as the alteration produced by the 
first 4 kilogs., 8'23 for the second, and 8'29 for the third, and an average on the whole 
of 8'27 for 4 kilogs. 
The wire was now further lengthened by ] 3'3 centims., and the vernier shifted so that 
the same length of wire as before was under examination, and again a rest of two days 
allowed. On loading the wire with the same weights as before, an alteration of 8'50 
was produced by the first 4 kilogs., 8'48 by the second, and 8'50 by the third, giving 
