Brief description

A technical drawing instrument that can be used for inscribing circles or arcs. As dividers, they are also used as a tool to measure distances, in particular on maps. It consists of two parts (legs) connected by a hinge which can be adjusted to change the radius of the circle drawn or the distance measured.

Use / Function

• Geometry: Drawing perfect circles and arcs.
• Navigation: Measuring distances on maps (using dividers).
• Carpentry: Marking measurements and scribing (transferring profiles).
• Masonry: Laying out stone patterns.

Operating principle

The compass works on the principle of a fixed radius. By fixing one leg (the needle point) and rotating the other leg (the drawing point) around it, a circle is created. The distance between the two legs determines the radius. Friction at the hinge keeps the legs at a constant angle.

How to create it

1. Legs: Shape two straight legs from wood, bone, or metal. They should be sturdy and straight.
2. Hinge: Connect the two legs at the top.
   • Simple: Tie them together tightly with cord/leather (lash compass).
   • Better: Use a rivet or a screw that allows movement but provides enough friction to hold the position.
3. Points:
   • Anchor Point: Sharpen one leg to a point or attach a metal needle.
   • Drawing Point: The other leg can also be sharp (for scratching/scribing) or have a holder for charcoal, lead, or ink.

Materials needed

• Legs: Hardwood, Bone, Brass, Iron, or Steel.
• Hinge: Leather thong, rivet, or screw.
• Tips: Hardened steel or iron for the points; charcoal or lead for drawing.

Variants and improvements

• Dividers: Both legs have sharp points. Used solely for measuring and transferring distances.
• Beam Compass: Two points slide along a beam (rod) to draw very large circles.
• Spring Compass: A spring bow holds the legs apart, and a screw adjusts the distance, offering greater precision.
• Calipers: Curved legs for measuring outside or inside diameters of objects.

Limits and risks

• Slip: The anchor point can slip if not pressed firmly, ruining the drawing.
• Accuracy: A loose hinge leads to inaccurate circles.
• Size: Limited by the length of the legs (unless using a beam compass).