How to Use a Geometric Compass

A compass is a handy drawing tool to have around. Use it to draw circles, make equal line segments or find the midpoint of a line.

Instructions

Difficulty: Moderately Easy

Things You’ll Need:

• Geometric Compasses
• Cardboard Or Magazine
• Pencil Sharpeners
• Pencils And Erasers
• Tacks
• String
• Rulers

To Draw a Circle

1. Insert a sharp pencil into the holder on the compass.
2. Open the compass to the radius desired for your circle. The radius is the distance from the center point of the circle to the outer edge.
3. Place the compass point on a piece of paper where you would like the center point of the circle to be.
4. Place the point of the pencil on the paper.
5. Rotate the compass to mark a circle on the paper with the pencil. Draw without lifting the point of the compass off the paper.

To Draw Equal Line Segments

6. Insert a sharp pencil into the holder on the compass.
7. Place the point of the compass on one end of the line segment you wish to duplicate.
8. Open or close the compass so that the pencil point is even with the other end of the line segment.
9. Draw a second line segment using a ruler. Make it a little longer than the segment you are duplicating.
10. Place the point of the compass on one end of the segment you have just drawn.
11. Draw an arc with the compass that intersects the line segment.
12. Erase the part of the line segment that goes beyond the pencil mark. The remaining line segment is equal in length to the line segment you wanted to duplicate.

To Find the Midpoint of a Line Segment

13. Insert a sharp pencil into the holder on the compass.
14. Place the point of the compass on one end of the line segment.
15. Open the compass so that the pencil reaches beyond the midpoint of the line segment.
16. Draw one arc above the line segment and another arc below the line segment.
17. Keep the compass open to the same setting.
18. Place the point of the compass on the other end of the line segment.
19. Draw arcs above and below the line segment. These arcs will intersect the first set of arcs you drew.
20. Use a ruler to draw a line from the point where the top arcs intersect, through the line segment, to the point where the bottom arcs intersect. The point where this line intersects the original line segment is the midpoint.

Tips & Warnings

• The diameter of a circle is the distance from one edge, through the center point, to the opposite edge of the circle. It is twice the length of the radius.
• The wider you set the opening in your compass, the larger your circle will be.
• Use a piece of string to draw very large circles. Tie a pencil to one end of the string. Use a tack to hold the other end in place. Holding the string taut, use the pencil to draw a large circle.
• The geometric term for finding the midpoint of a line segment is called "bisecting" the segment.
• Handle a compass carefully; it has a very sharp point.
• Place a magazine or a piece of cardboard underneath your paper to prevent the compass point from damaging your desktop.