That “flexible curve ruler” seems to be called a Lesbian rule (after the island; https://en.wikipedia.org/wiki/Lesbian_rule https://en.wikipedia.org/wiki/Lesbia

That “flexible curve ruler” seems to be called a Lesbian rule (after the island; https://en.wikipedia.org/wiki/Lesbian_rule https://en.wikipedia.org/wiki/Lesbian_rule), and can also be used to measure curve length (bend the rule so that is matches the curve to be measured, mark begin and end of the to-be measured length, straighten the rule, and use a straight ruler to measure the length between the marked points)