Use a small compass to identify a magnet’s poles: bring it near one end and watch which end of the compass needle responds. If the magnet end repels the needle’s north-seeking end, it is north; if it attracts that end, it is south. A freely suspended magnet or a labeled reference magnet can also help. Iron filings show a magnet’s field pattern, but not which pole is north.
Use a compass for a direct polarity test
- Place the magnet on a stable surface. Keep it away from speakers, motors, electrical equipment, other magnets, and large pieces of magnetic metal.
- Bring a small compass close to one end. A plotting compass is convenient, but a borrowed compass works; no special purchase is required.
- Observe the compass needle’s north-seeking end. Repulsion means the end of the magnet under test is north. Attraction means it is south.
- Repeat at the other end. The opposite end of a simple bar magnet has the opposite polarity. If the needle swings unpredictably, move away from possible interference and try again.
A compass needle is itself a small magnet that pivots to align with the local magnetic field, as the U.S. Geological Survey explains. The magnet being tested can determine the needle’s orientation when its field is stronger than Earth’s field at the compass, a qualitative condition described in PHYWE’s polarity experiment.
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What north-seeking means
The compass end commonly marked N is its north-seeking end: it points generally toward geographic north when used away from nearby disturbances. In polarity tests, “north” refers to the magnet’s pole label, not simply the direction an end points on a table. Opposite magnetic poles attract; like poles repel. That is why observing repulsion against the compass needle’s north end is a clear indicator that the tested end is also north.
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The terminology can be confusing when relating a compass to Earth’s field. The Institute of Physics’ magnet experiments notes the potential for confusion between Earth’s magnetic field and the labels on magnets. For the practical test, rely on the attraction-or-repulsion rule rather than trying to infer a pole label from geographic direction.
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Other ways to identify the poles
| Method | How to use it | What it tells you | Limitation |
|---|---|---|---|
| Freely suspended magnet | Suspend the magnet so it can rotate freely, then let it settle. | The end pointing generally north is the north-seeking pole. | It relies on Earth’s field and can be disturbed by nearby magnetic fields. A compass also needs freedom to rotate; compass balance can vary by hemisphere, as NOAA notes in its geomagnetism FAQ. |
| Known-polarity reference magnet | Bring a clearly labeled pole near the test magnet’s end. | Repulsion indicates like poles; attraction indicates opposite poles. | The reference label must be trusted. Attraction alone is not enough to name an unmarked pole unless the other pole is known. |
| Iron filings | Place paper or another suitable barrier over the magnet and sprinkle filings on top. | The filings reveal the field’s shape and where it is concentrated. | The pattern does not show field direction, so it cannot by itself distinguish north from south. Avoid getting filings directly onto a strong magnet because they can be difficult to remove. The OpenStax explanation of magnetic field lines describes how filings align with the field. |
Why attraction alone can mislead
An unmarked object may be attracted to either pole, so attraction by itself does not establish whether the tested end is north or south. Use the compass needle’s known north-seeking end and look for repulsion, or use a reference magnet with a reliable pole label. Iron filings are useful for mapping a field, but their pattern has no directional arrow; pair them with a compass if the goal is to label polarity.
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If the result is unstable
- Move the compass and magnet away from electrical equipment, magnetic materials, and other magnets. The USGS notes that electrical systems and local crustal magnetization can alter compass direction.
- Check that the compass needle can pivot freely and is not touching the magnet.
- Repeat the test from another position. A steady repulsion or attraction is more useful than a needle that repeatedly changes direction.
- If using a freely suspended magnet, ensure it can rotate without touching its support and allow it to settle before reading its direction.
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