Small Strains in Bars subjected to Twist, 523 



Tamed bar of Bessemer steel, length under measurement 

 = 10-25 inches, diameter = 0*747 inches, torsion-arm = 15 

 inches, a constant. 



The figures in the first column give the load in the pan at 

 the end of a constant arm. 



Load, 

 lbs. 



Reading. 



Differences. 









 26 

 51 



77 

 103 

 128 

 154 

 179 

 205 

 231 

 256 

 283 

 307 

 333 

 358 

 384 

 410 

 435 

 460 

 485 

 510 



26 

 25 

 26 

 26 

 25 

 26 

 25 

 26 

 26 

 25 

 27 

 24 

 26 

 25 

 26 

 26 

 25 

 25 

 25 

 25 



1 



2 



3 



4 



5 



6 



7 



8 .. ...... 



9 



10 



11 



12 



13 



14 



15 



16 



17 



18 



19 



20 





If / = length of the bar, 



d = diameter, 

 Tm = twisting-moment, 



= relative angular displacement, 



C = modulus of rigidity. 

 Then for bars of uniform circular section 



C = 



32TJ 

 TrdW 



and we have for this test-bar 



= 11,790,000 lb. per square inch. 



A second test gave almost identically the same results. 



The performance of the instrument is limited by the accuracy 

 of the micrometer-screw, and in the present instrument the 

 smallest angular displacement capable of measurement is about 

 four seconds of arc. As the contacting bar is not heavy, no 

 difficulty is experienced in balancing it, and therefore its length 

 may be considerable. This form of instrument is therefore 

 adapted for measuring the strains in long test-bars. 



The second form of apparatus differs from the preceding in 



