A uniform wire of length $l$ of weight w is suspended from the roof with a weight of W at the other end. The stress in the wire at $\frac{l}{3}$ distance from the top is $\left(\frac{\mathrm{W}}{\mathrm{A}}+\frac{2}{\gamma} \frac{\mathrm{w}}{\mathrm{A}}\right)$, where, A is the cross sectional area of the wire. The value of $\gamma$ is $\_\_\_\_$。