Make a Homemade Compass

A magnet that finds north

Long before phones and maps, sailors crossed whole oceans using a small spinning needle in a box: the compass. Amazingly, you can build a working compass at home using a needle, a magnet and a bowl of water. Let's find out how it works — and then make one.

What is a compass?

A compass is simply a small magnet that is free to spin. Every magnet has two ends, called poles — a north pole and a south pole. Earth itself acts like a giant magnet, with its own magnetic field stretching all around the planet. When a small magnet can turn freely, it swings until it lines up with Earth's field, so one end always points north. That is the whole secret.

You will need

• A steel sewing needle (the kind that is attracted to a magnet)
• A fridge magnet or a stronger bar magnet
• A small slice of cork, or a flat piece of foam or paper
• A bowl of water
• An adult to help with the needle

Safety first: the needle is sharp. Ask an adult to help, handle it carefully, and keep it away from younger children. No heat, fire or chemicals are needed for this experiment.

Step 1: Magnetise the needle

Hold the needle flat on a table. Take your magnet and stroke it along the needle in one direction only, from the eye to the point. Lift the magnet away at the end, move it back to the start, and stroke again the same way. Repeat this about 40 to 50 times, always the same direction.

This lines up the tiny magnetic parts inside the steel, turning your ordinary needle into a weak magnet.

Step 2: Float the needle

Lay the needle gently on top of the cork slice or a small flat piece of paper. Carefully place the cork on the surface of the water so it floats without sinking. Make sure it sits in the middle of the bowl and does not touch the edges.

Step 3: Watch it find north

Wait and watch. The cork will slowly turn until the needle settles, pointing in one steady direction. Spin it gently with your finger and let go — it swings back to the same line every time. Congratulations: your needle is pointing roughly north–south! Check it against a phone compass or a real one and you will see it is close.

Why does it happen?

The needle is now a magnet, and Earth is a magnet too. Magnets always try to line up with each other. Because the needle is floating, there is almost nothing to stop it turning, so even Earth's gentle magnetic pull is enough to swing it into line. The needle ends up pointing the same way as Earth's magnetic field: toward the north and south poles.

Earth has this magnetic field because deep inside it, far below the rocks we walk on, there is a layer of liquid iron that is always moving and swirling. That moving metal creates the planet's magnetism — like an enormous, invisible magnet buried in the core. You can explore this more in Earth's Magnetic Field and Inside the Earth.

Try this next

• Bring a fridge magnet close to your floating needle and watch it react. (Then take the magnet far away so it does not confuse the compass.)
• Magnetise the needle the opposite way and see if the same end now points south.
• Test what spoils your compass: try it near a phone, a radiator or a steel table.

Testing one change at a time like this is exactly how scientists work. To get better at it, read What Is a Fair Test? Variables Explained.

The big idea

A compass works because a free-spinning magnet always lines up with Earth's magnetic field. With nothing more than a needle, a magnet and some water, you have tapped into the same force that guided explorers across the seas for hundreds of years.