AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
85 
Critical points of tin and lead. 
The positions of the critical points of these metals have not been satisfactorily 
determined though several trials were made for that purpose, and in some of these 
great care was taken to load by very small amounts at a time and to allow each weight 
to remain on the wire for a considerable period; in the following experiment each 
load was left on the wire for 48 hours : — 
Experiment XX V. 
Annealed lead wire. 
Load in tenths of 
a kilog. 
Increase of resistance 
in divisions of the 
platinum-iridium wire. 
Ratio of consecutive 
increases of resistance. 
1st tenth .... 
7-0 
2nd. 
16-5 
2-4 
3rd. 
38-0 
2-3 
4th. 
49-0 
1-3 
5th. 
66-0 
1-4 
6th. 
103-0 
1-6 
7th. 
207-0 
2-0 
8th. 
540-0 
2-6 
9th. 
wire broke. 
If we regard the second load as the load at the first critical point, and the eighth as 
the load at the second critical point, we obtain for the loads at the two critical points 
respectively the values 48 kilogs. per square centimetre* and 74 kilogs. per square 
centimetre; in this case - would = 35 X 10 3 , and - =22 - 6xl0 3 , both these numbers 
« p 
being much larger than the values of - and — for the metals in Table VIII. 
« p 
With tin, also, the loads at the first and second critical points, as far as the latter 
could be determined,! seemed much less in proportion to the value of e than was the 
case with most of the other metals. 
In Table IX. the permanent increase per cent, of resistance produced by longitudinal 
traction is shown for each metal by a series of curves ; and these curves will also show 
very fairly the permanent increase per cent, of length produced, since the former is 
very nearly equal to twice the latter. 
* 30 kilogs. per square centimetre added for the permanent load, consisting of pulley, &c. 
t I am inclined, however, to attach very little value to the observations made on the critical points of 
lead and tin; very probably better results might be obtained by using wires of much greater section than 
those here employed. 
