Question
Describe the steps to make a simple magnetic compass using a needle, a magnet, and a cork floating on water. Explain the science behind why it works.
Solution — Step by Step
- A steel sewing needle (iron or steel needles work; stainless steel generally doesn’t magnetise well)
- A bar magnet or a refrigerator magnet (any reasonably strong permanent magnet)
- A small piece of cork (about 2 cm long)
- A bowl or plate of water
- Optionally: a reference compass to verify your result
Hold the needle on a flat surface. Stroke one end of the bar magnet along the needle, always in the same direction (from one end to the other), lifting the magnet between strokes rather than dragging it back.
Repeat 30–50 times. This magnetises the needle by aligning the magnetic domains (tiny magnetic regions in the steel) along the needle’s length.
Why stroke in one direction only? If you stroke back and forth, the domains align in one direction on the forward stroke and then partially re-align in the opposite direction on the back stroke — the net magnetisation is weak or zero. One-directional stroking progressively aligns more domains in the same direction.
Before making the compass, test if the needle is magnetised. Bring it near a paper clip or iron filing — it should attract them. Alternatively, suspend it freely from a thread: if magnetised, it will rotate until it aligns approximately north-south.
Push the magnetised needle through the cork horizontally — across its diameter — so the needle extends equally from both sides of the cork and the cork balances. The cork-needle assembly should be roughly symmetric.
Gently place the cork on the surface of the water in the bowl. Make sure there is no air current disturbing the water.
The cork will float, allowing the needle to rotate freely.
Within a minute, the needle will settle and point approximately north-south. The end that points toward geographic north is the north-seeking pole (the north pole of the needle magnet).
To identify which end is north: mark one end before magnetising, or use a reference compass, or use the fact that during magnetisation you can predict which end becomes which pole based on the pole of the magnet you used.
Why does the needle align north-south? Earth itself is a giant magnet. Its geographic north pole is near a magnetic south pole (this is why the north-seeking pole of a compass needle is attracted to Earth’s geographic north). The compass needle aligns with Earth’s magnetic field lines, which run roughly from geographic south to geographic north.
Why This Works
The compass works because of two principles:
1. Ferromagnetism and magnetic domains: Steel consists of microscopic regions called magnetic domains — each domain is like a tiny magnet. In unmagnetised steel, domains are randomly oriented, cancelling each other. Stroking with a permanent magnet aligns these domains in one direction, making the needle a permanent magnet.
2. Earth’s magnetic field: Earth generates a magnetic field due to electric currents in its molten outer core. This field extends through space and aligns compass needles. A freely suspended magnet experiences a torque (turning force) that rotates it until its north pole faces magnetic north. At equilibrium, the magnetic torque is zero and the needle is aligned with the field.
The floating cork is crucial — it provides near-frictionless rotation. A needle on a pin-point or hanging from a thread also works, but the cork-on-water method is simpler for a home experiment.
Alternative Method — Using a Pin as Pivot
Instead of cork and water, press a pin upright through a small piece of foam or clay on a flat surface. Balance the magnetised needle horizontally on the point of the pin — the needle can rotate freely around the pin. This works better in windy conditions than the water method (no ripples to disturb). The needle will still align north-south.
Another variation: tie thread around the middle of the needle (the balance point) and hang it freely — it will rotate horizontally until aligned with Earth’s field.
Common Mistake
The most common problem is stroking the magnet back and forth along the needle. This partially demagnetises the needle on the return stroke. Always stroke in one direction only — and lift the magnet completely off the needle before bringing it back to the starting position for the next stroke.
Another issue: using a stainless steel needle. Most stainless steel alloys are austenitic and non-ferromagnetic — they cannot be magnetised. Use a plain steel (high carbon steel) sewing needle for reliable results.
To remember which end of your homemade compass points north: stroke from the eye-end of the needle toward the sharp point using the south pole of the magnet. By the right-hand rule, the sharp-point end of the needle will become the south pole of the needle magnet (attracted to Earth’s magnetic north). Therefore, the eye-end will be the north pole of your compass needle (pointing south). Keep track of this when marking your compass.