187 
“ Notes of some Experiments made in February, 1881, on 
the Influence of Stress on the Electrical Kesistance of Iron 
and Steel Wires,” by Wm. H. Johnson, B.Sc. 
A piece of piano steel wire, about 0-040 inch diameter, 
was found to have a resistance of 0'418 Ohms. A heavy 
longitudinal stress causing a temporary elongation of 3 /, 
but not sufficient to produce any permanent strain, was 
applied, and the resistance observed was 0-422 Ohms, the 
temperature in both cases being the same. 
If the resistance per meter gramme of the wire had been 
unaltered by the stress, the observed resistance of the wire 
under stress should have been 0*443 Ohms, or some 5 °/ Q 
more than that observed. 
The resistance of hardened and tempered steel wire thus 
appears to decrease 5 °/ 0 under stress. 
Experiments were next made on hard iron wire, about 
the same diameter as above, 1*774 meters long and 0*2274 
Ohms resistance. Under longitudinal stress, causing a 
temporary elongation of 0*33 °/ 0 , the resistance was found to 
be 0*232 Ohms. Now, if the wire had been unaltered in 
resistance by stress, the observed resistance should have 
been 0-229 Ohms. Thus it appears that the resistance of 
hard or unannealed iron wire, unlike piano steel, is increased 
by stress some 1 J %. 
The figures denoting the percentage of change of resis- 
tance caused by stress are of course only approximate, but 
serve well to denote the direction of the change. 
In both sets of experiments the iron and steel wires 
returned to their original resistance when the stress was 
removed. 
The stress applied to the steel wire was many times that 
applied to the iron wire, so it is just possible that the steel 
wire under a small stress would have behaved like the iron 
wire and increased in resistance. The stress applied to both 
