Only a portion of the elongation in wire $\left(1\right)$ is due to the force in the wire. Part of the elongation results from the temperature increase.

Solve this expression for ${\mathrm{\sigma }}_1$:

$$\begin{gathered} {\mathrm{\sigma }}_1=\frac{{\mathrm{E}}_1}{{\mathrm{L}}_1}\left[{\mathrm{\delta }}_1-{\mathrm{\alpha }}_1∆{\mathrm{TL}}_1\right] \\ =\frac{200,000 \mathrm{N}/{\mathrm{mm}}^2}{600 \mathrm{mm}}\left[0.84 \mathrm{mm}-(12.0\times {10}^{-6}/{}^{\circ }\mathrm{C})\left({50}^{\circ }\mathrm{C}\right)(600 \mathrm{mm})\right] \\ =\mathbf{160}\mathbf{.}\mathbf{0}\mathbf{ }\mathbf{MPa} \end{gathered}$$