LINES OF FLUX

Physicists consider magnetic fields to be comprised of flux lines, or lines of flux. The intensity of the field is determined according to the number of flux lines passing through a certain cross section, such as a centimeter squared (cm²) or a meter squared (m²). The lines are not actual threads in space, but it is intuitively appealing to imagine them this way, and their presence can be shown by simple experimentation.
Have you seen the classical demonstration in which iron filings are placed on a sheet of paper, and then a magnet is placed underneath the paper? The filings arrange themselves in a pattern that shows, roughly, the “shape” of the magnetic field in the vicinity of the magnet. A bar magnet has a field whose lines of flux have a characteristic pattern (Fig. 1).
Another experiment involves passing a current-carrying wire through the paper at a right angle. The iron filings become grouped along circles

Fig. 1. Magnetic flux around a bar magnet.

centered at the point where the wire passes through the paper. This shows that the lines of flux are circular as viewed through any plane passing through the wire at a right angle. The flux circles are centered on the axis of the wire, or the axis along which the charge carriers move (Fig. 2).

Fig. 2. Magnetic flux produced by charge carriers traveling in a straight line.