{{short description|Type of compass}} thumb|Late 19th-century reduction compass The '''reduction compass ''' or '''proportional dividers''' is a geometry tool with asymmetric-length branches to allow performing homothetic transformations, reproducing a pattern while enlarging or reducing its size and conserving angles. It was invented by Fabrizio Mordente before 1567.<ref>{{cite web |title=Reduction Compass |url=https://www.mhs.ox.ac.uk/epact/catalogue.php?ENumber=46906 |website=Epact |publisher=Museum of the History of Science, Oxford |access-date=10 January 2022}}</ref>
== Use and principle == The reduction compass is made of two branches, with two points each. Several types exist, including the following:
* 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 determines the enlargement factor.<ref>{{cite web |title=Reduction compass |url=https://catalogue.museogalileo.it/object/ReductionCompass_n01.html |website=Galileo Virtual Museum |publisher=Museo Galileo - Institute and Museum of the History of Science |access-date=10 January 2022}}</ref> <gallery mode=packed widths=180 heights=180 style="line-height:130%" caption="Reduction compass variants"> File:Reduction compass-MHS 1880-IMG 3829-gradient.jpg|Simple reduction compass on display at the Musée d'Histoire des Sciences de la ville de Genève. File:Reduction compass-MHS 1914-IMG 3834-gradient.jpg|Sliding-pivot reduction compass by Nairne on London, 18th century. On display at the Musée d'Histoire des Sciences de la ville de Genève. </gallery>
The underlying principle is that of proportionality of the sides of two similar isosceles triangles.
The reduction compass was customarily used in geography to change the scale of maps.
==References== {{reflist}}
Category:Technical drawing tools Category:Drawing aids Category:Stonemasonry tools