82 
Proceedings of the Royal Society 
Length 
(mm.) 
Weight 
(grms.) 
d* 
(mm.) 
Resistance 
(standard 
copper wire 
=unity). 
Length 
(length be- 
fore stretch- 
ing=unity). 
Resistance 
(resistance 
before 
stretching 
=unity). 
Wire 1. 1 
777-62 
809-56 
2246 
5075 
198-8 
239’8 
1-4369 
1-5519 
1 
1-0411 
1 
1-0800 
Wire II. | 
830-82 
890-04 
1246 
6746 
234-9 
311-2 
1-5376 
1-7803 
1 
1-0713 
1 
1-1579 
Wire III. | 
660-86 
729-14 
1246 
7911 
114-7 
216-9 
1-2308 
1-4864 
1 
1-1033 
1 
1-2105 
These results agree, as might be expected, with those which 
Moussonf has published on steel, iron, and copper wires, in the 
fact that the resistance increases very much faster than the length. 
This must be the case unless there be a diminution of resistance, 
due to tension, sufficient to neutralise the increase of resistance due 
to decrease of the cross section of the wire. It is interesting to 
ask, then — Does the decrease in the diameter of the wire account 
for that part of the increase of its resistance which is not due to 
the increase of its length? The following table answers this ques- 
tion. The column headed “ calculated resistance ” contains the 
resistance as it ought to have been if its increase had been due only 
to change of dimensions; — 
Specific Gravity 
Observed 
resistance after 
stretching. 
Calculated 
resistance. 
Before stretching. 
After stretching. 
Wire I. 
10-4784 
10-5330 
1-080 
1-092 
Wire II. 
10-4967 
10-5646 
1-157 
1*155 
Wire III. 
10-5051 
10-5394 
1-210 
1-220 
The agreement of the figures in the observed and calculated 
columns is very close, notwithstanding the many sources of error 
to which the experiments were liable, such as the change in 
* See Wiedemann’s “ Galvanismus,” vol. i. p. 255. 
t “Galvanismus,” vol. i. p. 310; “Neue Schweizerische Zeitschrift, ” vol. 
xiv. (1855), p. 33. 
