ON THE MODULUS OF TORSIONAL RIGIDITY OF METAL WIl;i 45 



The numbers in the above table show considerable discrepancies in the value 

 found tor the rigidity modulus of any one substance by different observers. In 

 many cases this is no doubt due to want of purity and t<> imperfect annealing of the 

 sjjecimens used, also perhaps in some cases to the fact, that the wire was subjected to 

 excessive tension during the rigidity determinations. 



Owing to the fact that different observers have expressed the variation of rigidity 

 with temperature by different formulae (already quoted in Part 1. of this paper), it is 

 not easy to give a comparison of their results. It appeared to be best to calculate in 

 each case the average decrease in rigidity per 1 C. rise of temperature per unit 

 value of the modulus over the temperature C. to 100 (.)., this being the most 

 generally employed range of temperatures. In order to obtain numbers from the 

 present research to compare with those of other observers, a temperature coefficient 

 was calculated for each wire from the last set of observations at 100 C., and at the 

 ordinary lalxjratory temperature, assuming the rate of alteration of rigidity with 

 temperature to be constant in the interval. These numbers are thus the values 

 of a in the equation n/ = n, 5 [1 (<! 5)]. The coefficient ft the alteration 

 per 1 C. rise of temperature per unit value of the rigidity modulus, at 15 C. are 

 also included in Table XI. 



