Craft & DIY

#27 · Improvised compass arm

Tie a pencil to one end, hold the other end firmly in place, and swing an arc. Budget geometry tools that actually work.

A chopstick, a drawing pin and a pencil is a working compass in about thirty seconds - and unlike a school compass it will draw a circle of any radius you like, including ones far bigger than the instrument itself.

How to do it

1. Push a drawing pin through one end of the chopstick into a scrap of card taped under your paper, so the pin never touches the table.
2. Hold a pencil against the chopstick at the radius you want, or tape it there.
3. Keep steady downward pressure on the pin as you swing, or the arm will drift and the circle will spiral.
4. Sweep the pencil round in one continuous movement rather than in stages.

Why it works

A circle is defined as the set of points at a fixed distance from a centre. A rigid arm pivoting about a fixed point produces exactly that, which is why this works and why a compass is really just an arm with a nicer handle.

Tips & warnings

• Cut a shallow notch where the pencil sits so it cannot creep outwards mid-sweep - creep is what turns a circle into a spiral.
• Scaled up with a longer stick and a nail, this is how you mark out a circular flowerbed or a chalk circle on a driveway.