Definitions

drafting instrument

Compass (drafting)

A compass or pair of compasses is a technical drawing instrument that can be used for inscribing circles or arcs. They can also be used as a tool to measure distances, in particular on maps. Compasses can be used for mathematics, drafting, navigation, and other purposes.

Compasses are usually made of metal, and consist of two parts connected by a hinge which can be adjusted. Typically one part has a spike at its end, and the other part a pencil, or sometimes a pen. Circles can be made by fastening one leg of the compasses into the paper with the spike, putting the pencil on the paper, and moving the pencil around while keeping the hinge on the same angle. The radius of the circle can be adjusted by changing the angle of the hinge.

Distances can be measured on a map using compasses with two spikes. The hinge is set in such a way that the distance between the spikes on the map represents a certain distance in reality, and by measuring how many times the compasses fit between two points on the map the distance between those points can be calculated.

Compass and straightedge construction is used to illustrate principles of plane geometry. Although a real pair of compasses is used to draft visible illustrations, the ideal compass used in proofs is an abstract creator of perfect circles. The most rigorous definition of this abstract tool is the "collapsing compass"; having drawn a circle from a given point with a given radius, it disappears; it cannot simply be moved to another point and used to draw another circle of equal radius (unlike a real pair of compasses).

Variants

Beam-compass is an instrument with a wooden or brass beam and sliding sockets, or cursors, for drawing and dividing circles larger than those made by a regular pair of compasses.

Scribe-compass is an instrument used by carpenters and other tradesmen. Some Compasses can be used to scribe circles, bisect angles and in this case to trace a line. It is the compass in the most simple form. Both branches are crimped metal. One branch has a pencil sleeve while the other branch is crimped with a fine point protruding from the end. The wing nut serves two purposes, first it tightens the pencil and secondly it locks in the desired distance when the wing nut is turned clockwise.

Loose leg wing dividers are made of all forged steel. The pencil holder, thumb screws, brass pivot and branches are all well built. The dividers are somewhat rusted and were produced at an unknown date. They are used for scribing circles and stepping off repetitive measurements with some accuracy.

As a symbol

A compass is often used as a symbol of precision and discernment. As such it finds a place in logos and symbols such as the Freemasons' Square and Compasses and in various computer icons. John Donne uses the compass as a conceit in "A Valediction: Forbidding Mourning" (1611).

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