Pedro, a contractor, is trying to choose between purchasing a steel or an aluminum $50-ft$ tape measure. $($a$)$ If he wants the tape measure that expands and contracts less, which tape measure should he purchase? Consider that the tape measures were calibrated at $68\mathrm{.}0F$ in the factory.

$($b$)$ How much can he expect his new tape measure to contract if the temperature outside is $18\mathrm{.}5F?$