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When a wire is stretched longitudinally, it contracts laterally. Poisson's ratio (nu) quantifies this coupling: nu = -$\frac{lateral strain}{(longitudinal strain)}$ = -$\frac{Delta d/d}{(Delta L/L)}$. It is dimensionless with a theoretical range of -1 to 0.5.

Practical values: steel (0.29), copper (0.34), glass (0.25), rubber (0.49-0.50), cork (approximately 0). These values have direct practical significance. Cork (nu approximately 0) is ideal for bottle stoppers — it doesn't expand laterally when compressed into the neck. Rubber (nu approximately 0.5) is nearly incompressible — stretching it lengthwise causes proportional lateral contraction to conserve volume.

The volume change during stretching depends on Poisson's ratio: Delta V/V = $epsilon_L$ * (1 - 2*nu). If nu = 0.5, Delta V = 0 (incompressible). If nu < 0.5, volume increases on stretching. Materials with negative nu (auxetic materials) expand in all directions when pulled — these are engineered metamaterials.

The upper limit nu = 0.5 arises from the thermodynamic requirement that bulk modulus must be positive. If nu exceeded 0.5, a material would expand under pressure — physically unstable.