Volumetric thermal expansion coefficient
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The volumetric thermal expansion coefficient (sometimes simply thermal expansion coefficient) is a thermodynamic property of a substance given by

β = - (1 / ρ)(∂ρ/∂T)ρ const

where

ρ  density
T  Temperature 

β measures the fractional change in density as temperature increases at constant pressure.

The original article can be found at: http://en.wikipedia.org/wiki/Volumetric_thermal_expansion_coefficient.
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