Stress and Strain on the. Properties of Matter. 



105 



hi torsional oscillation with a load in the pan, S, slightly greater than 

 any which it was intended to use, and after each set of oscillations a 

 long rest was allowed. The object of this preliminary treatment* was 

 to extend the limit of elasticity as much as possible. 



Again, immediately before the testing the wire was set in torsional 

 oscillation, but not through a greater arc than that through which it 

 was intended eventually to twist the wire.f Finally, the loads which 

 were intended to be used in the pans, T, were put in and taken out 

 ten or a dozen times, and then the actual trials began, the load in the 

 pan, S, having been some time previously reduced to the amount to 

 be used in the trial. The following experiment will give a fair 

 notion of the degree of accuracy which was obtained. 



Experiment X. 



A hard-drawn aluminium wire, about '800 cm. long, and 0*1 cm. in 

 diameter. The load on the wire was merely that of the scale-pan, 

 i.e., 2 kilos. 



Load in each pan 

 producing torsion. 



Position of index. 



Degrees of torsion 

 produced by the 

 torsional stress. 







grms. 



230 70 





20 



5L70 



179 -00 







>> 



233-93 



182 23 



20 



>> 



50-95 



182 '98 







5> 



23120 



180 '25 



20 





51-70 



179 -50 







» 



233 -90 



182 20 



20 



5> 



52-80 



181-10 







J> 



/ 233 -05 



180 -25 



20 



>> 



5173 



181 32 







>> 



233 05 



181 -32 







181-02 



The probable error of the mean value 181*02, given above, is 

 0*14 per cent. 



A set of observations was next made with 10 grams instead of 

 20 in the pans, T, the mean number of degrees of torsion being in 

 this case 90*43, with a probable error of 0*36 per cent. Within the 

 limits of probable error 90*43 is the half of 181*02. In calculating 



* The period of this treatment depends upon the nature of the metal ; with iron 

 it is advisable that it should extend over a couple of days. 



f The object of this will be seen from the author's paper on 11 The Internal 

 Friction of Metals," 'Phil. Trans.,' vol. 177 (Part II), 1886. 



i 2 



