These lines are a way to show the structure of a magnetic field. A compass needle will always point along a field line. The lines are close together where the magnetic force is strong, and spread out where it is weak.

Mathematical construction: a family of curves in space giving at each point the direction (by the tangent of the line) and the strength (by the local density of lines) of a magnetic field.

The trajectories along which magnetic particles would align, or charged particles would flow, if placed in a magnetic field.

lines in space, used for visually representing magnetic fields. At any point in space, the local field line points in the direction of the magnetic force which an isolated magnetic pole at that point would experience. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Imaginary lines that indicate the strength and direction of a magnetic field. The orientation of the line and an arrow show the direction of the field. The lines are drawn closer together where the field is stronger. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.

A magnetic field has both a strength and a direction at each point in space. For example, at each point on the earth, the magnetic field -- and thus a compass -- points a particular direction, roughly toward the North. Magnetic fields are therefore generally represented as lines: the direction of the line gives the direction of the field, and the number of lines indicates the strength.

lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.