and Poisson's Ratio by Method of Flexures. 



the longitudinal curvature, then Young's Modulus 

 given by 



209 



(E) was 



E 



M 3 



/M 



> 



where /> 1 = breadth and d = depth of the cross-section. 



The writer used the above method extensively during the 

 war to determine values of Young's Modulus and Poisson's 

 Ratio for timber. It was found capable of measuring very 

 small changes of curvature. For instance, the value of 

 Poisson's Ratio for spruce corresponding with lateral strain 

 in the direction of the grain is about 0*01, so that the lateral 

 curvature is only about To - s of the longitudinal curvature. 

 It was, however, possible to measure the lateral curvature in 

 such cases by using a micrometer instead of a scale, and 

 placing it about 150 in. from the mirrors, instead of about 50 in. 

 as was usually the case. By this means it was found possible 

 to measure changes in curvature corresponding with strains 

 of about 1 in 5 x 10 6 . 



There are a few evident precautions necessary when con- 

 ducting an experiment, such as avoiding longitudinal thrust 

 on the specimen by properly suspending the knife-edges, 

 and preventing bodily rotation of the specimen during test. 



Also the knife-edges were slightly 

 curvature was free to take place. 



In Table I. are given two sets 

 Modulus (E) and Poisson's Ratio 



Table I. 



curved, so that lateral 



of values of Young's 

 (a) obtained for five 



Elastic 

 Coefficient. 



Type of 

 Experiment. 



Steel. 



Wrought 

 Iron. 



Brass. 



Copper. 



Aluminium. 



E (10 6 lb./sq. in.). 



Flexure. 



28-1 



27-9 



14-2 



17-6 



9 23 



E (10 6 lb./sq. in.). 



Tensile 

 (Marten's 

 Instrument). 



28-3 



28-3 



140 



17-9 



930 



! 



\M{\W lb./sq. in.). 



Torsion. 



11-2 



10-8 



5-41 



6-80 



349 



i a 



Flexure. 



0236 



0-245 



0333 



0-305 



0313 



(7 



Calculated 

 from rows 1 

 and 3. 



0-255 



0-290 



0-310 



0-295 



0*320 





E=Young's Modulus. ct = Poisson's Modulus. M=z Modulus of Rigidity. 



Phil. Mag. S. 6. Vol. 41. No. 242. Feb. 1921, P 



