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Updated on April 14, 2010

A compass is a 2-legged instrument for locating a point or describing an arc. The legs are pointed and meet at a joint. They are adjustable so that the distance between the points can be varied.

Draftsmen use the term "compass" only for an instrument in which one of the points is a pencil or pen used for drawing an arc. They apply the term "dividers" to the compass when it is equipped with two sharp points (without pen or pencil) for use in stepping off equal units of space on their drawings or for transferring measurements from one sheet to another. Compasses, usually in the style of the draftsman's dividers, are essential tools for many other craftsmen also, including carpenters, machinists, pattern makers, tinsmiths, and toolmakers. As used by such craftsmen, one point serves to establish the center point and the other to scribe an arc or mark a point by scratching the working surface. In mathematics, the compass and ruler are used to construct geometrical figures.

Photo by Kriss Szkurlatowski
Photo by Kriss Szkurlatowski

In the plain-hinge style of compass, the legs retain their relative positions because of friction at the joint. A handle, extending above the hinge, may be provided to permit easy and rapid turning of the compass. Ingenious arrangements have been devised to ensure that the handle will bisect the angle between the legs. Friction-joint dividers are sometimes provided with a flat spring device in one leg to provide a minute adjustment bf the angle between the legs.

To extend the capacity of the ordinary compass by 2 or 3 inches (5.1 to 7.6 cm), provision is made for the insertion of an extension bar in one leg. Compass legs more than 5 inches (12.7 cm) long usually are provided with a knee in each leg to permit setting the points at right angles to the working surface, no matter what angle the legs may make. Such a device is essential when using a pen to ensure that both blades of the pen will make proper contact with the surface.

When a drawing requires much work to be done from a single center, there is a tendency for the center point of the compass to make a sizable hole, causing the compass leg to be positioned off center. This disadvantage can be avoided by means of a small transparent disk fixed in position on the drawing by three sharp points mounted on the metal rim of the disk. A depression in the upper surface of the disk receives the compass point, which can be placed repeatedly at the precisely desired center point.

Compasses were found in the ruins of Pompeii, but they were used by mathematicians centuries before the eruption of Vesuvius buried Pompeii in 79 A.D. Early records of compasses antedate the writings of Ahmes (1650 B.C.).

Photo by Alaa Hamed
Photo by Alaa Hamed

Special Types of Compasses

Bow pen compasses are instruments with spring-opened legs. In one design, the two legs of the compass are joined at the upper ends by a hinge, and the compass is equipped with a separate spring to open the legs. In another design, the legs are formed as integral flat springs from a single piece of metal. In both designs, the tendency of the springs to open the legs is restrained by a screw that controls the opening between the legs. In bow instruments, the handle always bisects the angle between the legs.

A drop compass is a special form of short-radius bow compass for drawing many small circles rapidly. It is useful, for example, in structural drawings where many rivets are represented.A beam compass is for use where relatively large distances are involved. It is essentially a bar with two movable members attached. One member carries a point for the center; the other carries a point, pencil, or pen, depending on the intended use. The two members can be slid along the bar to the desired positions.

A bullet compass has one leg with an enlarged, round end shaped like a bullet. It is used for shop work where arcs are often scribed around a central hole instead of around a point.

A very useful form of compass is the proportional divider, used for enlarging or reducing in a desired ratio. Its two legs cross, forming an X, and there are points at all four ends. The joint, which is movable, can be shifted at will to selected positions. Thus the distance between the two points at one end will be in a fixed ratio to the distance between the two points at the other end. This permits transferring distances measured 'at one end to a proportionately greater or shorter distance at the other end.


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