A bimetallic strip is created by rivetting together two identical...

When the bimetallic strip is heated, both the copper and steel strips expand.

The increase in length is given by $\Delta L = L_0 \alpha \Delta T$.

Since the coefficient of linear expansion of copper is greater than that of steel ($\alpha_{copper} > \alpha_{steel}$), the copper strip expands more than the steel strip.

To accommodate the unequal expansion while remaining riveted together, the bimetallic strip bends.

The strip with the greater expansion (copper) will form the outer curve, which has a larger radius and greater length. Thus, the bimetallic strip bends with copper on the convex side.