A compass, more accurately known as a pair of compasses, is a

technical drawing Technical drawing, drafting or drawing, is the act and discipline of composing drawings that visually communicate how something functions or is constructed. Technical drawing is essential for communicating ideas in industry and engineering ...

instrument that can be used for inscribing

circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is con ...

s or arcs. As

dividers A caliper ( British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the dimensions of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital d ...

, it can also be used as a tool to mark out distances, in particular, on

map A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes. Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...

s. Compasses can be used for mathematics, drafting,

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

and other purposes. Prior to computerization, compasses and other tools for manual drafting were often packaged as a set with interchangeable parts. By the mid-twentieth century, circle templates supplemented the use of compasses. Today those facilities are more often provided by computer-aided design programs, so the physical tools serve mainly a didactic purpose in teaching

geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ...

, etc.

Construction and parts

Compasses are usually made of metal or plastic, and consist of two "legs" connected by a hinge which can be adjusted to allow changing of the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

of the circle drawn. Typically one leg has a spike at its end for anchoring, and the other leg holds a drawing tool, such as a

pencil A pencil () is a writing or drawing implement with a solid pigment core in a protective casing that reduces the risk of core breakage, and keeps it from marking the user's hand. Pencils create marks by physical abrasion, leaving a trail ...

, a short length of just pencil lead or sometimes a

pen A pen is a common writing instrument that applies ink to a surface, usually paper, for writing or drawing. Early pens such as reed pens, quill pens, dip pens and ruling pens held a small amount of ink on a nib or in a small void or cavity wh ...

Handle

The handle, a small knurled rod above the hinge, is usually about half an inch long. Users can grip it between their pointer finger and thumb.

Legs

There are two types of leg in a pair of compasses: the straight or the steady leg and the adjustable one. Each has a separate purpose; the steady leg serves as the basis or support for the needle point, while the adjustable leg can be altered in order to draw different sizes of circles.

Hinge

The screw through the hinge holds the two legs in position. The hinge can be adjusted, depending on desired stiffness; the tighter the hinge-screw, the more accurate the compass’s performance. The better quality compass, made of plated metal, is able to be finely adjusted via a small, serrated wheel usually set between the legs (see the "using a compass" animation shown above) and it has a (dangerously powerful) spring encompassing the hinge. This sort of compass is often known as a "pair of Spring-Bow Compasses".

Needle point

The needle point is located on the steady leg, and serves as the center point of the circle that is about to be drawn.

Pencil lead

The pencil lead draws the circle on a particular paper or material. Alternatively, an ink nib or attachment with a

technical pen A technical pen is a specialized instrument used by an engineer, architect, or drafter to make lines of constant width for architectural, engineering, or technical drawings. " Rapidograph" is a trademarked name for one type of technical pen. T ...

may be used. The better quality compass, made of metal, has its piece of pencil lead specially sharpened to a "chisel edge" shape, rather than to a point.

Adjusting nut

This holds the pencil lead or pen in place.

Uses

Circles can be made by pushing one leg of the compasses into the

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distrib ...

with the spike, putting the pencil on the paper, and moving the pencil around while keeping the legs at the same

angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the '' vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...

. Some people who find this action difficult often hold the compasses still and move the paper round instead. The

of the intended circle can be changed by adjusting the initial angle between the two legs. Distances can be measured on a map using compasses with two spikes, also called a dividing compass (or just "dividers"). 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.

Compasses and straightedge

Compasses-and-straightedge constructions are used to illustrate principles of

plane geometry Euclidean geometry is a mathematical system attributed to ancient Greek mathematician Euclid, which he described in his textbook on geometry: the '' Elements''. Euclid's approach consists in assuming a small set of intuitively appealing axioms ...

. 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).

Euclid Euclid (; grc-gre, Εὐκλείδης; BC) was an ancient Greek mathematician active as a geometer and logician. Considered the "father of geometry", he is chiefly known for the '' Elements'' treatise, which established the foundations of ...

showed in his second proposition (Book I of the '' Elements'') that such a collapsing compass could be used to transfer a distance, proving that a collapsing compass could do anything a real compass can do.

Variants

beam compass A beam compass is a compass with a beam and sliding sockets or cursors for drawing and dividing circles larger than those made by a regular pair of compasses. The instrument can be as a whole, or made on the spot with individual sockets (called ...

is an instrument, with a wooden or brass beam and sliding sockets, cursors or trammels, for drawing and dividing circles larger than those made by a regular pair of compasses. Scribe-compasses is an instrument used by carpenters and other tradesmen. Some compasses can be used to draw 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. A wing nut on the hinge 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. They are used for scribing circles and stepping off repetitive measurementsFine Woodworking, Laying out dovetails, Chris Gochnour, pg. 31, The Taunton Press, No. 190, April 2007 with some accuracy. A

proportional compass The sector, also known as a proportional compass or military compass, was a major calculating instrument in use from the end of the sixteenth century until the nineteenth century. It is an instrument consisting of two rulers of equal length joi ...

, also known as a military compass or

sector Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a p ...

, was an instrument used for calculation from the end of the sixteenth century until the nineteenth century. It consists of two rulers of equal length joined by a hinge. Different types of scales are inscribed on the rulers that allow for mathematical calculation. A

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 ...

is used to reduce or enlarge patterns while conserving angles.

As a symbol

A pair of compasses 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 icon In computing, an icon is a pictogram or ideogram displayed on a computer screen in order to help the user navigate a computer system. The icon itself is a quickly comprehensible symbol of a software tool, function, or a data file, accessible on ...

s. English poet John Donne used the compass as a

conceit An extended metaphor, also known as a conceit or sustained metaphor, is the use of a single metaphor or analogy at length in a work of literature. It differs from a mere metaphor in its length, and in having more than one single point of contact bet ...

in " A Valediction: Forbidding Mourning" (1611). File:Scribe line A.jpg, Compass for tracing a line. File:Scribe compass A.jpg, Flat branch, pivot wing nut, pencil sleeve branch of the scribe-compass. File:Loose leg wing dividers.jpg, 6 inch (15 cm) dividers made from forged steel. File:Compas de proportion 1.jpg, One type of sector. File:Coat of arms of East Germany.svg, A compass on the former

National Emblem of East Germany The national emblem of East Germany featured a hammer and a compass, surrounded by a ring of rye. It was an example of what has been called " socialist heraldry". It was the only heraldic device of a European socialist state with a ring of grai ...

(former German Democratic Republic). File:Masonic silver pendant.jpg, The compass is a Masonic symbol that appears on jewellery such as this pendant.

References

External links

Beam or trammel compass
(variant form) {{Authority control Mathematical tools Navigational equipment Stonemasonry tools Technical drawing tools