AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
39 
Receiver! January 18,—Read January 26, 1882. 
PART II.-ELECTRICAL CONDUCTIVITY. 
The Temporary Alteration of Electrical Conductivity produced 
by Longitudinal Traction. 
W. Thomson, in 1856,* investigated the effects of tension on electrical conductivity 
in copper and iron wires, and, moreover, stated that he “ had very nearly established, 
for the case of iron at least, that the augmented resistance due to tension, either 
temporary or permanent, is a very little more than can he accounted for by the change 
of form.” 
In 1877+ I determined in absolute units the amount of alteration produced by 
longitudinal traction in the resistance of steel, iron, and brass, and proved that the 
temporary alteration of resistance resulting from increase of length and diminution of 
section is with iron and steel about two-fifths and with brass four-fifths of the 
whole observed change. 
In the above-mentioned experiments I experienced considerable difficulty in obtain¬ 
ing accurate results in consequence of the minuteness of the changes to be measured, 
and, therefore, set about devising some plan whereby the variations of resistance caused 
by slight changes of temperature, which had proved a source of great annoyance, 
might be eliminated. In this attempt I have been entirely successful, and with the 
arrangements described below have determined, with I believe considerable accuracy, 
the very small changes of resistance which can be temporarily produced by mechanical 
tension. 
Description of apparatus. 
Pieces, about 7-§- feet in length, of the same wires as used in Part I. were suspended 
in pairs, as in fig. 6|, in an air chamber 4 feet in height, 4 inches inner diameter and 
6 inches outer diameter; the inner of the two concentric cylinders of which the air 
chamber was composed being surrounded by a layer of water 1 inch thick enclosed 
between the two cylinders. This vessel, which was made of tinned iron, rested on a 
table, provided with a suitable aperture, ft, and was furnished with two thermometers, 
T T, passing through the outer cylinder and into the axis of the inner one. The ends 
of the two wires were clamped into three short and stout brass blocks, A, B, C, which 
•Phil. Trans., Part IV., Feb. 28th, 1856, §§ 150-152. 
t Proc. Roy. Soc., No. 183, 1877. 
X In this figure the key employed to close the battery circuit is placed near the Leclanche cell and not 
in its actual position near the scale and lamp, in order to show the connexions more clearly. 
