Definition of proportional compass in US English:

proportional compass

noun

  • Chiefly historical a compass having two opposite pairs of legs turning on a common pivot, the position of which is adjustable, so that the distances between the pairs of points on either side of the pivot may be set to any desired ratio.

Origin

Late 16th century; earliest use found in John Dee (1527–1609), mathematician, astrologer, and antiquary.