ON THE MODULUS OF TORSION AL RIGIDITY OF METAL WIRES. 43 



was quite constant, and with silver the value of the modulus at the temperature of 

 the laboratory, in the last f't-w experiments, was unaltered by a temporary increase of 

 temperature. The tin wire was the only case in which the rigidity modulus at the 

 ordinary laboratory tempo at HI was lessened by heating.] 



6. The internal viscosity of all the metals examined, with the exceptions of soft 

 iron and steel, increases with the temperature. This increase varies very much with 

 different metals, being greatest with aluminium and least with platinum. The 

 internal viscosity of soft iron decreases rapidly with rise of temperature, and reaches 

 a minimum value at al>out 100 C. There is a slight decrease also in the case of 

 steel. 



7. Repeated heating and continual oscillation through small amplitudes decrease 

 the internal friction. 



8. Both the internal friction and the period of torsional vibration increase with the 

 amplitude of oscillation. The increase is generally greater the higher the tem- 

 perature of the wire. It is least in the case of steel and is small in the case of soft 

 iron. 



9. Vibration through a large amplitude considerably alters both the logarithmic 

 decrement and period of oscillation at smaller amplitudes. The nature of the 

 alteration varies with different metals, being in some cases an increase and in some 

 a decrease. 



10. The internal viscosity of a well annealed wire suspended and left to itself 

 gradually decreases. 



11. The internal viscosity of an unaunealed wire is enormously reduced by 

 annealing. 



The values of the modulus of torsional rigidity obtained in the present research 

 are collected together in Table X., which also contains the values given by the 

 observers mentioned in Part 1. of this paper, who have investigated the variations 

 of the modulus with changes of temperature. 



2 



