Carrying a length
To cut four equal stool legs, a carpenter cuts one. That leg is then held against each new piece.
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The rule
Segment AB has two ends, points A and B.
Step 1 of 4
- A segment from A to B.
- Compass point on A, pencil on B. The span is set.
- Lift the compass. Put the point down at C.
- Swing the pencil. The curved mark is an arc.
The span is the gap between the compass point and the pencil.
Every point on the arc is one span from C. The span is the length of AB.
A ruler gives only the nearest mark. A length between 46 and 47 mm cannot be read exactly. The mm means millimetres. The compass keeps the exact length.
Example
A new line has a point C on it. Set the span on AB.
Put the compass point on C. Draw an arc across the new line.
Call the crossing D. Now CD is a copy of AB.
In the lab, drag the pencil until it sits on B. Then tap Carry to C. If CD is not a copy, drag again.