AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
107 
It will be noticed also that the curves like those of Tables XV. and XVI. are of 
parabolic shape. 
The Alteration of Specific Gravity produced by Permanent Extension, 
Hammering, and Permanent Torsion. 
In calculating the values of the specific resistance after the wire had been subjected to 
any one of the above-mentioned processes, it was in the first instance assumed that there 
was no change in the specific gravity of the substance, and afterwards a correction applied 
for such change. The specific gravity of the wires in the annealed condition having been 
determined, the change wrought by extension, hammering, or torsion carried to an extent 
which was about three-fourths of that which the substance would bear without rupture 
was determined by a very delicate balance, and though it was not possible to dislodge 
any air bubbles which might be attached to the metal when immersed in water by boiling 
for fear of partially annealing the specimens, these bubbles were brushed off very 
carefully and considerable pains were taken in the weighings. A very fine platinum 
wire was used for the purpose of suspending the substances in water, and the vessel 
employed for holding the water was of considerable size so as to avoid the necessity of 
bending the wire to any great extent. The coils in which the wire under examination 
was wound, when weighed, were kept together by fine copper wire, and the weight of 
this in air and water together with that of the fine platinum wire was from time to 
time determined. The next experiments show that the change in specific gravity is 
roughly proportional to the amount of strain, provided this strain is not continued up 
to the point of rupture. 
Experiment XXX VII. 
Wire of annealed silver, 8 feet in length and ’090 centime in diameter, stretched 
by loading. 
Increase of length 
per cent. 
-A. 
Decrease of specific 
gravity per cent. 
= B. 
Ratio of B : A. 
115 
•0124 
•on 
3-64 
•0461 
•013 
7-28 
•0892 
•012 
9-38 (broke) 
T465 
•016 
P 
o 
