\(\mu = \frac{\Delta \delta}{\Delta l}\), where \(\Delta \delta\) is the lateral contraction and \(\Delta l\) is the elongation.
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\(\mu\)
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The Poisson Ratio is the negative ratio of transverse to axial strain. In fact, when a sample object is stretched (or squeezed), to an extension (or contraction) in the direction of the applied load, it corresponds a contraction (or extension) in a direction perpendicular to the applied load. The ratio between these two quantities is the Poisson's ratio.