78 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
employed than with longitudinal stress, and that the alteration of specific resistance 
is much greater for the former kind of stress than for the latter, the ratio being 
about 2*85 : 1. 
The small alterations of resistance which can be produced by fluid pressure as shown 
in this table prove also how impossible it would have been to detect with such changes 
of pressure as can be effected by an air-pump any alteration of resistance due directly 
to compression, and therefore the value of the mode of experimenting already alluded 
to in determining the amount of lowering of the temperature of the freezing-point of 
water by pressure : in fact, the change of resistance due to the lowering of the tem¬ 
perature of the melting-point of ice by the pressure would be more than 22 times the 
change of resistance due to the pressure only. 
A brief consideration also suffices to show that the alteration of resistance due to 
any change of temperature is in all cases very much greater than that which would 
follow from the same change of volume produced by mechanical stress; this will be 
seen at once from a glance at the next table. 
Table VII. 
Name 
of 
mefal. 
Coefficient of 
cubical expansion 
at 20° C. 
Increase of 
resistance per unit 
caused by a rise 
of 1° C. at 20° C. 
Bise of 
temperature 
necessary to 
double the volume 
= A. 
Increase of 
resistance per unit 
caused by rise of 
temperature 
A = B. 
Increase of 
resistance per unit 
if the wire could 
have its volume 
doubled by 
mechanical stress 
= C. 
Eatio 
of 
B : C. 
Lead . 
■8223x10- 4 
•00375 
0 C. 
12,160 
45'6 
0-962 
47'4 
Copper . 
•4554 xlO- 4 
•00380 
21,960 
83'5 
0-904 
92-4 
Iron . 
■3420 x 10- 4 
•00470 
29,250 
137-5 
1-393 
987 
Brass . 
•5450 x 10- 4 
•00122 
18,350 
22-4 
0‘746 
30-3 
We see from this last table that the alteration of resistance due to any change of 
temperature is in the case of the four metals, lead, copper, iron, and brass, from 30 to 
nearly 100 times as great as that which would follow from the same change of volume 
produced by mechanical agency; and it would appear, again, therefore, that the 
increase of resistance caused by rise of temperature is principally due to other 
causes than mere expansion. 
Permanent Alteration of Resistance produced by Longitudinal Traction. 
Limit of elasticity. 
Experiments on the permanent alteration of resistance produced by traction possess 
considerable advantages in determining the limit of elasticity over the methods 
usually adopted. In the first place, it is possible to detect much more minute elonga¬ 
tions, even though very small lengths of the wire be employed, than would be the 
