Reduction compass

Late 19th-century reduction compass

The reduction compass is geometry tool with asymetric-length branched, as to allow performing homothetic transformations, that is reproducing a pattern while enlarging or reducting its size and conserving angles.

Use and principle

The reduction compass is made of two branches, with two points each. Several types exist:

• simple reduction compass : the pivot is fixed, and the enlargement factor is determined by the length difference between the small and long end of the branches
• mobile-headed reduction compass: the pivot is mounted on a slide, and its position determins the enlargement factor

Simple reduction compass on display at the Musée d'Histoire des Sciences de la ville de Genève. 

Sliding-pivot reduction compass by Nairne on London, 18th century. On display at the Musée d'Histoire des Sciences de la ville de Genève. 

The underlying principle is that of proportionality of the sides of two similar isocele triangles.

The reduction compass was customally used in geography to change the scale of maps.