A magnet inside a bigger magnet
How Does a Compass Work?
Earth is a giant magnet, and a compass needle is a small one lining up with it.
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Step 1 of 5
Earth generates a magnetic field.
Molten iron churning in the outer core produces electric currents, and those currents make a magnetic field that reaches far into space.
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Step 2 of 5
The needle is a small magnet, free to turn.
Balanced on a near-frictionless pivot, it can swing freely. A magnet in a field always rotates to line up with it.
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Step 3 of 5
Alignment, not attraction.
The needle is not being pulled towards the pole. It is rotating until it matches the field direction where it sits, which happens to be roughly north.
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Step 4 of 5
Magnetic north is not true north.
The magnetic pole sits some distance from the geographic one and drifts every year, so maps list a correction for your location.
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Step 5 of 5
Nearby metal ruins it.
A phone, a car body or a steel desk distorts the local field far more strongly than Earth does. Compasses need clear space to be trusted.
Earth's magnetic field has reversed many times in the past, and the rock record shows every flip.
The short version
A compass needle is a small pivoted magnet that rotates to align with Earth's magnetic field.
Try it yourself
Hold a compass, then bring a phone near it. The needle swings — proof it follows the strongest local field.
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