Magnetic field — this is the region around a magnet where other magnets or ferromagnetic materials experience magnetic force. The field is invisible but can be visualized using power lines — imaginary curves that emerge from the pole N, wrap around the magnet outward and enter the pole S. Line density shows field strength: where lines are denser (near the poles), the field is stronger.

Iron filings on the plane are tiny ferromagnetic particles. Each one gets magnetized in the magnetic field and returns along the direction of local force lines, so their arrangement 'draws' the shape of the field — a classic school experiment.

Two magnets placed next to each other: if facing one another various poles (N-S) — attract each other, lines of the field between them 'merge'; if co-named (N-N or S-S) — repel, lines bend and do not connect.

Compass — this is a small freely rotating magnet (arrow). It always orients itself along the total magnetic field at the point where it is. Drag it with the mouse (holding Ctrl) to explore the direction of the field in different locations.

Earth's magnetic field — the planet itself is a huge magnet (due to the movement of molten iron in its core). This weak, nearly homogeneous field forces compasses to point roughly north when they are far from other magnets.