AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
81 
Experiment XXII. 
A well-annealed silver wire was suspended for trial with the scale-pan attached, and 
loads increasing' by 1 kilog. at a time were put on for three minutes and then 
removed, when the permanent increase of resistance was determined for each load. 
Load in kilogs. 
Increase of resistance 
in divisions of 
platinum-silver wire. 
1st 
40 
2nd 
40 
3rd 
45 
4th 
60 
•5th 
75 
6tli 
80 
First critical point . 7th 
142 
8th 
186 
9th 
306 
Second critical point 10th 
1436 
11th 
2706 
Each division of the platinum-silver represents in this case an increase of resistance 
of only yyYo P er cent., and as the percentage of permanent elongation is half of this 
fraction, because the wire is decreased in section very nearly in the same proportion as 
it is increased in length, it follows that the total elongation produced by all the loads 
up to the first critical point inclusive, does not exceed '2 per cent. 
The case of iron. 
The behaviour of iron under longitudinal traction is very remarkable ; this metal, if 
the load be applied in small quantities at a time, is seen at certain points to become 
perfectly rigid, so that the further application of stress does not produce any further 
permanent elongation until the load has reached a certain value, when elongation once 
more commences.* Further, iron possesses three critical points at least, and may be 
found to have more;t these points are well shown in Experiment XXIII. 
* Tins was first noticed by J. T. Bottomley (Proc. Roy. Soc., No. 197, 1879), wbo kindly lent me 
some of the same kind of wire as tkat used by bimself. 
t Perhaps otker metals may have more than two critical points, which might be discovered by loading 
the wires more gradually. 
MDCCCLXXXIII. 
M 
