The transverse deformation in tension and compression
The experiment with a rubber bar shows that the cross dimensions of the section are decreasing in tension and increasing in compression. It is typical of tension and compression of all materials. Experimentally it was determined that the relation between the transverse and longitudinal deformations is constant for the material in tension and compression. The relation between transverse ε ' and longitudinal ε deformation was first stated by the French scientist Poisson (1781—1840). The dependence has the following form:
where
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is the ration of longitudinal deformations or Poisson, s ratio, 
is a dimensionless value. Poisson’s ratio depends on the material alone as well as the modulus of elasticity and it characterises its elastic property. Poisson’s ratio is considered the same in tension and compression.
