The compass and Earth's field

A compass needle is a tiny magnet that is free to turn. Its N end swings to point along B, the field at the spot where the needle sits. So a compass is a field-direction detector.

Far from nearby magnetic sources, Earth's field dominates. Its strength and tilt vary with location. A flat compass follows the horizontal component; this bench simplifies Earth's field to 50 µT horizontally northward. On the bench, north is away from you (up the screen) and east is to the right. Bring a magnet close and its field wins, because near a magnet B is far stronger than 50 µT.

Here is a famous puzzle. The N end of a compass points to Earth's geographic North, and unlike poles attract. So the magnetic pole up in the Arctic must be a magnetic south pole. Earth's geographic North is near a magnetic S pole, and its geographic South is near a magnetic N pole.

The name “north pole” of a magnet is short for north-seeking pole: the end that turns to face geographic north.