Representing magnetic fields: in practice
Physics Narrative for 11-14
With iron filings or with plotting compasses
A simple way of mapping the rough shape of a magnetic field is to use iron filings. For example, the magnetic field pattern of a bar magnet can be detected by covering it with a piece of paper and scattering iron filings over the paper.
As the filings are scattered around the magnet they become temporary magnets (by magnetic induction) and line up end-to-end. The filings tend to clump together around the poles of the magnet, indicating that this is where the magnetic field is strongest. The lines of iron filings give an impression of the lay-out of the magnetic field, but it is important to remember that what you have before you with the iron filings is not a magnetic field pattern. As argued earlier, the magnetic field pattern is a theoretical construction, a kind of graph which can be drawn to plot the layout of magnetic fields.
An alternative way to map a magnetic field is to use one or more plotting compasses. The plotting compass itself consists of a very small, suspended magnetic needle. If the plotting compass is placed in the magnetic field of a bar magnet, the needle will line up such that its north pole is attracted to the south pole of the magnet. By placing the plotting compass in adjacent positions in the field it is possible to map out some magnetic lines of force.
Using a plotting compass allows the direction of the field to be identified, with the needle of the compass pointing along the field lines from north to south. The plotting compass technique, however, gives less impression of the strength of the magnetic field than the iron filings technique, where the filings clearly clump around the poles.