When researchers map the three-dimensional flow of a river around a bridge pier, or of wind around the wing of an airplane (picture below), they use streamlines, lines that trace the flow of particles of water or air.



Magnetic field lines similarly describe the structure of magnetic fields in three dimensions.They are defined as follows. If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal--such needles exist, see bottom of page) then the needle will always point along the field line (drawing below).

Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other, and of course, the magnetic force is strongest near the poles where they come together. The behavior of field lines in the Earth's magnetic field is very similar.

Not so in space, however, where magnetic field lines are fundamental to the way free electrons and ions move. These electrically charged particles tend to become attached to the field lines on which they reside, spiralling around them while sliding along them, like beads on a wire (drawing below).

Because of this attachment, the behavior of electrified gas ("plasma") in space, a gas of free ions and electrons, is dictated by the structure of field lines: Electric currents, for instance, find it easiest to flow along such lines. Indeed, the role of field lines in a plasma resembles that of grain in wood: just as the grain is the "easy" direction along which wood splits most readily, so the direction of field lines is the one along which particles, electric currents, heat and certain types of waves prefer to flow.

A small bar magnet, on gimbals that allow it to point in any direction in space, can be procured from its manufacturer, Cochranes of Oxford, Ltd., Leafield, Oxford OX8 5NT, England. Two types are available, Mark 1 with jewelled bearings for $36.60, Mark 2 with simple bearings for $12.65. For details see their web site:
    http://www.cochranes.co.uk/secondary.html (scroll down to "Magnaprobe").