$$\begin{gathered} 20-\mathrm{mm} \mathrm{diameter} \mathrm{steel} \mathrm{rod} \\ \mathrm{A}=314.16 {\mathrm{mm}}^2 \\ \mathrm{E}=200 \mathrm{GPa} \\ \mathrm{\alpha }=12.0\times {10}^{-6}{/}^{\circ }\mathrm{C} \end{gathered}$$

Since the rod is connected to immovable supports, the contraction that would have resulted from a temperature decrease is counteracted by a tension force developed in the rod. The magnitude of this tension force is such that it causes the rod to elongate exactly the same amount as it would have contracted if it had been allowed to deform freely.