Editing Poisson's ratio (section)

=== Width change ===
[[Image:Rod diameter change poisson.svg|350px|thumb|right|Figure 2: The blue slope represents a simplified formula (the top one in the legend) that works well for modest deformations, {{math|∆''L''}}, up to about ±3. The green curve represents a formula better suited for larger deformations.]] 
If a rod with diameter (or width, or thickness) {{mvar|d}} and length {{mvar|L}} is subject to tension so that its length will change by {{math|Δ''L''}} then its diameter {{mvar|d}} will change by:
:<math>\frac{\Delta d}{d} = -\nu \frac{\Delta L} L</math>

The above formula is true only in the case of small deformations; if deformations are large then the following (more precise) formula can be used:
:<math>\Delta d = -d \left( 1 - {\left( 1 + \frac{\Delta L} L \right)}^{-\nu} \right)</math>
where
*{{mvar|d}} is original diameter
*{{math|Δ''d''}} is rod diameter change
*{{mvar|ν}} is Poisson's ratio
*{{mvar|L}} is original length, before stretch
*{{math|Δ''L''}} is the change of length.

The value is negative because it decreases with increase of length