Another experiment involves passing a current-carrying wire through the paper ata right angle, as shown in Fig. 8-3. The iron filings will be grouped along circles centeredat the point where the wire passes through the paper.Physicists consider magnetic fields to be comprised of flux lines. The intensity ofthe field is determined according to the number of flux lines passing through a certaincross section, such as a square centimeter or a square meter. The lines don’t really ex-ist as geometric threads in space, or as anything solid, but it is intuitively appealing toimagine them, and the iron filings on the paper really do bunch themselves into lineswhen there is a magnetic field of sufficient strength to make them move.Magnetic polarityA magnetic field has a direction at any given point in space near a current-carryingwire or a permanent magnet. The flux lines run parallel with the direction of the field.A magnetic field is considered to begin at the north magnetic pole, and to terminate atthe south magnetic pole. In the case of a permanent magnet, it is obvious where thesepoles are.Magnetic Polarity1378-2Pattern of magnetic flux lines around a bar magnet.