STRAIN is defined as the ratio of CHANGE in dimension to the original dimension.If ‘δl’ is the change in the length and ‘l’ is the original length, strain = δl/lThe ratio of change in length to the original length is called the longitudinal strainThe ratio of change in diameter/breadth to original diameter/breadth is called the lateral strainThe ratio of lateral strain to longitudinal strain is called the POISSON's ratio∴ Poisson's ratio is μ defined as the ratio of lateral strain to longitudinal strain.\(\rm{\mu = - \frac{{{\epsilon_{lateral}}}}{{{\epsilon_{longitudinal}}}}}\)For ALUMINIUM μ = 0.334, for Cork μ=0, for Rubber μ=0.5