Flux lines

Perhaps you have seen the experiment in which iron filings are placed on a sheet of paper, and then the magnet is placed underneath paper. The filings arrange themselves in the pattern which shows, roughly, the shape of the magnetic field in the vicinity of the magnet. A bar magnet has a field having characteristic form.

Figure--         Pattern of magnetic flux lines around a bar magnet.

Another experiment involves passing a current-carrying wire through the paper at a right angle, as shown in the Figure. The iron filings will be grouped along the circles centered at point where the wire passes through paper.

The Physicists consider magnetic fields having flux lines. The intensity of the field can be determined according to the number of flux lines passing through a certain cross section, such as a square centimeter or a square meter. The lines do not exist as geometric threads in space, or as anything solid, but it is intuitively appealing to imagine them, and iron filings on the paper do bunch themselves into lines when there is a magnetic field of enough strength to make them move.