STRAIN ON THE PHYSICAL PROPERTIES OF MATTER. 
825 
Experiment XIII. 
The copper wire used in Experiment IX. was treated in the same manner as the 
steel in the last experiment, except that now the cylinders Cb were dispensed with. 
The temperature throughout remained nearly constant at 20° C. 
Vibration period in 
seconds. 
Load on Ihe wire in 
kilo?. 
Couple in dyne 
centims. required to 
twist the wire 
through one radian. 
1-891 
0-534 
46539 
3-327 
4-534 
46841 
4-312 
8-534 
46818 
5-105 
12-534 
46708 
5-801 
16-534 
46817 
Remarks on Experiments XII. and XIII. 
Experiment XII. shows that for the unannealed piano-steel the couple required to 
twist the wire through one radian is quite independent of the load for loads ranging 
from 4 to 20 kilos. For smaller loads than 4 kilos, the value of the couple is slightly 
less. With the annealed copper, the couple required to twist the wire through one 
radian is independent of the load for all the loads. 
Having thus endeavoured to prove that the torsional elasticity does not depend 
upon the load when the latter is comparatively large, both for hard-drawn and 
annealed wires, I will consider the effect of light loading. As regards annealed wire, 
Experiment XIII. furnishes evidence that the elasticity is very nearly independent of 
the load, even for light loads, for the lightest load in this experiment does not much 
exceed -g kilo A Care must, however, be taken, with annealed wire, to begin with the 
heaviest load first, as loading, if not carried to the extent of causing perceptible 
permanent elongation, invariably produces an increase of elasticity, which is partly 
permanent and partly sub-permanent, and this last must not be disturbed by shocks 
in unloading if we are to strictly compare the values of the torsional elasticity with 
different loads.! 
As regards the unannealed piano-steel, the case is otherwise, for if the loads be 
small the torsional elasticity may vary very perceptibly with them. 
* This would produce only a very small stress per square centimetre as compared with the stress used 
with the unannea-led piano-steel. 
f See similar remarks respecting the internal friction. With the heavier loads still remaining on the 
wire, the sub-permanent elasticity is not so readily shaken out as with the lighter ones. I have since 
thoroughly satisfied myself that the torsional elasticity is independent of the load, even for light loads, in 
the case of annealed wires, provided sufficient time be allowed after changing each load. 
5 N 
MDCCCLXXXVI 
