ELASTIC MODULI. 



Young's Modulus.* 



Table 79. 



• The Young's Modulus of elasticity is used in connection with elongated bars or wires of elastic material. It is 

 the ratio of the number representing the longitudinal stress per unit of area of transverse section to the number rep- 

 resenting the elongation per unit of length produced by the stress, or : — 



Young-s Modulus:^ ^"1?"^'^ °^ longitudinal stress . 

 Elongation per unit length. 



In the case of an isotropic substance the Young's Modulus is related to the elasticity of form (or rigidity modulus) 

 and the elasticity of volume (or bulk modulus) in the manner indicated in the following equation : — 



where E is Young's Modulus, n the rigidity modulus and i the bulk modulus. 



The bulk modulus is the ratio of the number expressing the intensity of a uniform normal stress applied all over 

 the bounding surface of a body (solid, liquid or gas) to the number expressing the change of volume, per unit volume, 

 produced by the stress. 



t The modulus for cast iron varies greatly, not only for different specimens, but in the same specimen for different 

 intensities of stress. It is diminished for tension stress by permanent elongation. 



t See also Table 72. 



Smithsonian Tables. 



75 



