AND STRAIN ON THE ACTION OE PHYSICAL FORCES. 
109 
gravity of copper was decreased by the torsion more than 2 per cent.—a large amount 
considering the small alteration 4 ' which extension and hammering can produce. 
The results recorded in the last table enabled the previously mentioned correction 
for change of specific gravity to be made in calculating the specific resistance. 
For if A 1 and Ao be the specific gravities respectively before and after stretching, 
hammering, or twisting, and S and x be respectively the specific resistances un¬ 
corrected and corrected for change of specific gravity, »=SXp. 
Now the table furnishes the means of determining 
and 
since a is very small 
Therefore 
The correction though small was applied in all cases. 
Effect of Cooling Suddenly on the Specific Resistance of Steel. 
We have seen in Part I. that the effect of suddenly chilling steel heated to a high 
temperature produces a somewhat similar effect on the elasticity to that of excessive 
permanent extension, and it was concluded to be highly probable that whether the 
distance between the molecules be increased by mechanical strain or by the strain 
caused by sudden cooling, the elasticity in the direction of the line of separation of 
the molecules diminishes to a minimum as the separation increases, and then begins to 
increase. Now BarusI has proved that the specific resistance of steel increases 
continuously with its hardness, but Barus’s experiments were made with steel 
heated at or above a visible red, and as the strain produced by extension, ham¬ 
mering, and torsion had been shown to produce up to a certain point decrease of 
resistance, it seemed a matter of some interest to ascertain whether heating the steel 
to a lower temperature than that of dull red and then cooling slowly would not also 
have the effect of decreasing the specific resistance. The following experiment was 
therefore tried :— 
A] A 3 
= a. say ; 
A, 1 -II , 
— =~— = l + a very nearly, 
A, 1—a J J 
r=S X (1 -pa). 
* Not so much as 1 per cent, in any case which I have examined, 
t Phil. Mag., November, 1879. 
