62 Dr 8earU, A bifilar method of measuring the rigidity of ivires 



axis through is T .A'B' .ON/l. Since the moment due to the two 

 tensions equals that of the appHed couple, G dyne-cm., 



G^2T. A'B'. ON/l. 

 But A'B' .ON is twice the area OA'B' and thus is fz/Zg sin 6. We 

 thus obtain 



_^ _ A'B'. ON _ a^a2 sin d 



Mg- 



h 



h 



Fig. 2. 



Since h = {P - A'B'^}^, we see that, when A'B' is small compared 

 with I, we may put h = I, and so obtain 



G=''-^^^.Mg. 



•(i; 



In the examples of § 7, ^ never differed from I by as much as 

 1 in 4000. 



