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Use a small compass to identify a magnet’s poles: bring it near one end and watch the compass needle. If the magnet end repels the needle’s north-seeking end, that magnet end is north; if it attracts it, that end is south. You can also let the magnet rotate freely and see which end points generally north. Iron filings reveal a magnet’s field pattern, but cannot label its poles on their own.
Use a compass for a direct polarity test
- Move away from interference. Keep the compass away from speakers, motors, electrical equipment, and other magnets. Nearby electrical systems and magnetized rock can affect a compass reading, as the U.S. Geological Survey explains.
- Bring the compass near one end of the magnet. A plotting compass or other small magnetic compass is suitable; no special instrument is required.
- Watch the compass needle’s north-seeking end. If that end is repelled by the magnet end you are testing, the test end is north. If it is attracted, the test end is south.
- Repeat the observation. If the needle wavers or the result changes, move farther from potential interference and test again.
A compass needle is itself a small magnet that pivots and aligns with the local magnetic field. The USGS describes it as “a small magnet, one that is allowed to pivot freely” (USGS, April 20, 2008). The tested magnet can turn the needle when its field is stronger than the surrounding field; PHYWE describes this as the usual situation for a permanent magnet, without specifying a numerical strength threshold (PHYWE experiment instructions).
Other ways to identify the poles
Let the magnet rotate freely
Suspend the magnet near its center with a thread, or otherwise support it so it can turn with little resistance. After it settles, the end pointing generally toward geographic north is its north-seeking pole; the other end is south. This uses Earth’s magnetic field as the reference, so nearby magnetic objects, electrical equipment, or constraints on the magnet’s rotation can change the alignment. A compass also needs to rotate freely to align properly, and NOAA notes that compass balance designs differ between hemispheres; this matters especially when using a compass outside the hemisphere for which it was balanced (NOAA geomagnetism FAQs).
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Compare it with a magnet whose pole is labeled
Bring a trusted, labeled pole near the unmarked end. Repulsion means the poles are alike; attraction means they are opposite. Repulsion is the clearest confirmation because like poles repel. Attraction by itself cannot identify the unknown pole unless the reference pole’s label is reliable.
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Use iron filings to map the field, not name the poles
Place paper or another suitable barrier over the magnet, then scatter filings on top to reveal the field pattern. The filings align along the field but do not show its direction, so the pattern alone cannot tell you which end is north. Add a compass if you want to assign north and south. IOP Spark describes this paper-and-filings demonstration and other magnet-polarity activities (IOP Spark: Experiments with magnets); OpenStax explains that magnetic field direction is defined by the direction a north pole would move (OpenStax: Magnetic Fields, Field Lines, and Force). Keep filings off a strong magnet because they can be difficult to remove.
Why “north-seeking” does not mean geographic north pole
The compass needle end called north is the end that points generally toward geographic north when the compass is free to turn. The naming can be confusing because opposite magnetic poles attract: the north-seeking end is drawn toward the region near geographic north, rather than being repelled by it. For identifying a handheld magnet, avoid inferring its polarity from geography alone; use the observed compass repulsion or attraction, or a trusted labeled magnet.
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