What it shows:
The magnetic field lines of the Earth can be represented by the field lines of a bar magnet.
How it works:
The Earth's magnetic field is basically a magnetic dipole. It can therefore be represented to first approximation by the field of a bar magnet. The shape of the field lines can be highlighted by the sprinkling of iron filings, or by the use of plotting compasses. The latter method has the advantage of showing the variation of dip angle with latitude, with the lines of force running parallel to the surface of the Earth at the equator, and normal to the Earth's surface at the magnetic poles. It could be noted that the average magnetic field strength of the Earth (50 µT = 0.5 Gauss) is several hundred times weaker than the field around the bar magnet.
figure 1. Idealized sketch of the Earth's magnetic field. The orbiting compasses show the varying dip angle.
The first suggestion that the Earth was like a giant bar magnet was by William Gilbert (1544-1603), physician to Queen Elizabeth I. The Earth's magnetic field is created by electric currents generated by the rotating iron/nickel core. Its magnetic moment is 6.4×1021Am2. The actual magnetic field lines are distorted from the symmetrical dipole shape by the Solar wind, which confines it to a region called the magnetosphere. A useful accompaniment to this would be our Earth Globe to show the location of the magnetic poles (the North geomagnetic pole near Vostok, Antarctica; the South geomagnetic pole on Ellef Ringnes Island, north Canada).
1. C.R. Carrigan, D. Gubbins Scientific American, February, 1979, pp.92-101