Steel rails are laid at a temperature of ${\text{50}}^{\text{0}}\text{F}$ . The gap that must be left between each standard 10 m rail section and the next if the rails should just touch when temperature raises to ${122}^{\text{0}}\text{F}$ ${\text{(α=12×10}}^{\text{-6}}{\text{/}}^{\text{o}}\text{C)}$ is