CUBIC ELASTICITY. 



23 



This last becomes of use in experimental work, where T, I, 

 it, and d are determined from measurements taken, when 

 D is possible to calculate the value of G for the material in 

 question. 



For hollow shafts, this becomes, 



- 2 l T (XIV.) 



32 n 



where R and r are the outer and inner radii, and D and 

 d the corresponding diameters. 



12. Cubic Elasticity. This, or as it is often called, 

 elasticity of volume, relates to changes, not of length, 

 breadth, or distortion, but to changes of volume. If a 

 body be subjected to a stress normal to its surface at every 

 point, its volume will be changed, and either increased or 

 diminished. If the original volume of a body is V, and a 

 pressure p be applied to its surface, the volume will be 

 changed by an amount v when 



- = ^ (XV) 



V v' 



K being the co-efficient of cubic elasticity. 



FIG. 10. 



Consider the case of a cube acted upon by a 

 uniform stress p, on every face. If L is the length of 



