ON THE MODULUS OF TORSIONAL RIGIDITY OF METAL WIRES. 37 



found with this wire will \te found in Part I. of this paper. Some experiments were 

 made to determine the manner in which the period and logarithmic decrement varied 

 with the amplitude of vibration of the vibrator. The results are given in Table IX. 



TABLE IX. Steel Wire. 



From these it is seen that the logarithmic decrement of the vibrations is less at the 

 higher temperature than at the lower one. This was also found to be the case with 

 soft iron. It is also seen that the period increases only very slightly with the 

 amplitude so long as this is small. The logarithmic decrements are slightly greater 

 at the larger amplitudes, but the increase is not uniform. 



Silver. 



A pure silver wire was used, and it was found that the rigidity modulus for silver 

 is much more nearly constant at a constant temperature than for most of the other 

 metals examined. This was particularly evident in the case of the observations 

 taken at the ordinary temperature, which were not very seriously altered by the 

 successive heatings, especially in the case of those taken near the end of the series. 

 There was, however, on the whole a slight increase of rigidity with time. 



The rigidity-temperature curve for silver is given in Diagram VII. The points 

 marked represent the first and second oliservations at the ordinary temperature of 

 the room and the means of the observations at the higher temperatures. The line is 

 slightly curved in the same direction as that for platinum (see Diagram IV.). 



The value of the modulus of rigidity for silver at 15 C. was found to be 

 n, 6 = 2-6741 X 10" dynes per sq. centim., and of the "temperature coefficient" 

 /3 = '0004540. 



The logarithmic decrements of the amplitudes of vibration when plotted against 

 the corresponding temperatures gave a curve similar to that obtained tor copper 

 (Diagram II.). 



