112 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
XVT., and XVII. on the alteration of specific resistance produced by strain, shows 
most conclusively the superiority of platinum-silver to German-silver in making 
standard resistance coils. 
We have seen also in Part I. that rest, materially in some cases and to a certain 
extent in all, increases the elasticity of metals ; now German-silver wire furnishes a 
conspicuous example of this; and the curve in the table representing' the increase of 
torsional rigidity produced by rests, shows plainly that the restitution of elasticity and 
electrical conductivity go hand in hand. And this circumstance, when taken in con¬ 
junction with the fact that there is no change in the dimensions of the wire which 
would at all account for the increase of elasticity or conductivity, evidently teaches 
us that when we can increase the elasticity without altering the mean molecular 
distance, we at the same time increase the electrical conductivity. 
The Influence of Permanent Strain on the Change of Electrical 
Conductivity produced by Alteration of Temperature. 
Permanent extension, hammering, and torsion. 
The previous observations on the change of specific resistance produced by extension, 
hammering, and torsion having given rise to the suspicion that these processes caused 
the wires to alter their susceptibility to change of resistance from change of tempera¬ 
ture, and that this alteration of susceptibility bore a marked relation to the alteration 
of the thermo-electrical properties which are also caused by stress and strain, it w 7 as 
considered very advisable to further investigate the matter. 
In the first few trials the large air chamber previously described was employed, but 
the changes wrought in the coefficient of increase of resistance from rise of tempera¬ 
ture by the various strains were found to be so small that special precautions had to be 
taken to secure reliable results. Accordingly the following apparatus was made use of. 
In the figure Z Z is an air chamber made of two concentric brass tubes 12 inches in 
length. The diameter of the wider tube is 4-| inches, and of the inner tube 2i inches, 
so that the two enclose between them a layer of air 1 inch thick. The two tubes are 
connected at the top and bottom by brass rings soldered to them, and two pieces of 
brass tubing soldered into the outer cylinder, one near the bottom and the other near 
the top, communicate by means of indiarubber tubing with a boiler, W, and a vessel 
of cold water, K, respectively, so that steam from the boiler can fill the entire space 
between the two concentric cylinders and be condensed in Iv. Three stout wires of 
copper, S S S, 4 inches in length, pass through a cork which fits tightly the inner 
cylinder, and their upper ends are connected with the resistance coils and galvanometer, 
whilst to their lower ends are attached three clamps, A A and B. To the clamps are 
soldered silk-covered copper wires, which, passing through the cork, are connected with 
the poles of the battery. The ends of the wire to be tested, X, and the comparison- 
