Getting Accurate Constructions From a Compass

Compass constructions are marked on accuracy, and most lost marks come from four mechanical problems rather than from not knowing the method.

The Compass Slips

The most common failure. The point wanders during the arc, so the radius changes and the arc does not meet where it should.

The fix is pressure on the point rather than on the pencil, and rotating the compass by the knurled handle at the top rather than by pushing the pencil leg.

Working on a slightly soft surface, with a few sheets of paper underneath, lets the point bite rather than skate.

The Hinge Is Loose

If the compass opens or closes during use, the radius is not constant and nothing will line up.

Tighten the hinge screw until the legs move firmly rather than freely. A compass that flops open is the single most common reason a construction fails, and it is a thirty-second fix.

The Lead Is Wrong

• Too soft: the line is thick and imprecise
• Too blunt: the effective radius changes as it wears
• Not level with the point: the radius is wrong from the start
• Too long or short: the compass sits at an angle
• A hard lead, sharpened to a chisel edge, is the standard answer

Sets that include a spare lead and allow the pencil leg to be adjusted, of the kind at eboot.net, make this straightforward rather than a matter of hoping the supplied lead lasts.

Draw Full Arcs

A habit that gains marks. Short tick marks frequently miss the intersection, and examiners want to see the construction.

Generous arcs that clearly cross, left visible rather than rubbed out, demonstrate the method and are easier to work from.

Do Not Rub Out the Construction

The most commonly lost easy marks. Construction arcs are evidence of method, and erasing them removes exactly what is being assessed.

Leave them. A construction question is asking how you got there rather than only where you got to.

The Constructions Worth Knowing

Bisecting a line segment. Bisecting an angle. Dropping a perpendicular from a point to a line. Constructing a perpendicular at a point on a line. Copying an angle.

Those five underpin nearly every construction question, and each is a small number of steps once the compass behaves.

Check by Measuring

A construction should be done with compass and straight edge, and there is no rule against checking the result with a ruler and protractor afterwards.

A bisected angle that measures noticeably unequal halves indicates the compass slipped, and it can be redone.

Handle the Point Carefully

Compass points are sharp enough to cause real injury and are the usual source of minor accidents in a classroom.

Point cover on when not in use, kept in a case rather than loose in a bag, and never carried point-upward.