336 On the Modulus of Torsional Rigidity of Wires. [Mar. 17, 



temperature between 10° C. and 100° 0. is constant for pure copper 

 and for steel, but not for any of the other materials examined. 



3. In the case of the metals iron, gold, tin, lead and commercial 

 copper, the rigidity temperature curve is of such shape as to suggest 

 that if it were possible to obtain the values of the rigidity modulus 

 at the different temperatures, all within a very short space of time 

 (so as to avoid the " time effect "), the resulting curve would be a 

 straight line. 



4. In the case of the metals platinum, silver, aluminium, the shape 

 of the rigidity-temperature curve shows that the effect of the gradual 

 increase of rigidity with time is such as to make the alteration with 

 temperature approximate more closely to a linear law than would be 

 the case if the observations were all taken within a very small interval 

 of time. For these metals, therefore, the decrease of torsional 

 elasticity per 1° C. rise of temperature increases with the temperature. 



5. In general, the effect of heating to a high temperature is to 

 increase the value of the rigidity modulus at lower temperatures. 

 (This applies even to pure copper, of which the modulus of rigidity at 

 the ordinary laboratory temperature is slightly greater after the wire 

 has been heated to higher temperatures. The rigidity of steel was quite 

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

 of the laboratory in the last few experiments was unaltered by a 

 temporary increase of temperature. Tin was the only case in which 

 the rigidity modulus at the ordinary laboratory temperature 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 about 100° C. There is a slight decrease also in the case of steel. 



7. Repeated heating and continued oscillation through small ampli- 

 tudes 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 temperature 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 unannealed wire is enormously 

 reduced by annealing. , 



