AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
77 
Table VI. 
Name of 
Metal. 
Decrease of 
resistance per 
unit produced 
by an increase of 
fluid pressure of 
1 grm. per 
square centim. 
= A. 
Increase of 
resistance per 
unit produced by 
a longitudinal 
tension of 
1 grm. per 
square centim. 
= B. 
.Ratio 
of 
B: A. 
Decrease of 
resistance per 
unit attending 
a fluid pres¬ 
sure sufficing 
to halve the 
length of the 
wire. 
Increase of 
resistance per 
unit attending 
longitudinal 
tension suffic¬ 
ing to double 
the length of 
the wire. 
Decrease of 
specific resis¬ 
tance per unit 
attending a 
fluid pressure 
sufficing to 
halve the 
length of 
the wire 
= C. 
Increase of 
specific resis¬ 
tance per unit 
attending 
longitudinal 
tension suffic¬ 
ing to double 
the length of 
the wire 
= D. 
Ratio 
of 
C : D. 
Lead. 
10638 x 10~ 13 
17310 xlO- 13 
1-63 
2-440 
2-885 
3-440 
1-613 
2-14 
Copper . 
1257-0 xlO- 13 
2310-Ox IO- 1 3 
1-84 
3-470 
2-713 
4-470 
1-005 
4’45 
Iron . 
1160-0 xlO- 12 
21111 x 10-12 
1-82 
5-269 
4-180 
6-269 
2-618 
2-39 
Brass 
1064-0 x 10- 12 
2265-5 x IO- 1 3 
213 
3-004 
2-239 
4-004 
166 
2-41 
Means . 
1-83 
2-85 
Explanation of and remarks on Table VI. 
A few words are necessary on the methods of calculating the numbers given in 
columns five and seven. 
If e be the coefficient of longitudinal elasticity, and <x the ratio of lateral contraction 
to elongation, it can easily be proved that if we subject a wire to a fluid pressure of 
1 grm. per square centimetre, the decrease in length per unit thereby produced will be 
1 _ 2a- 
——. If then A denote the decrease per unit of resistance produced by the pressure, 
it follows that the decrease attending pressure which would suffice to halve the length 
of the wire would be A-r--——, and in this way the numbers in column five have been 
determined from those in column two; the values of e and cr being those given in 
Part I., with the exception of the value of cr for brass, which has been taken from 
Mallock’s paper,'" as I have reason to believe that the values of cr obtained by me 
for this metal are too large. Again, since the pressure would for such small amounts 
as are used here cause a decrease of section which would be double the decrease of 
length, the effect of the pressure in merely altering the dimensions would be to in¬ 
crease the resistance by 1 per unit. In order, therefore, to deduce the values in 
column seven from those in column five we have only to increase the former by 1. 
It will be noticed that the total alteration of resistance produced by the fluid pressure 
is in all cases less than the alteration produced by the same amount of longitudinal 
stress, the ratio of the latter alteration being to that of the former as 1'83 : 1 ; but 
that the alteration of resistance when the same change of length is produced by the 
two kinds of stress is, except in the case of lead, greater when fluid pressure is 
* Proc. Royal Society, June, 1879. 
