In 1600, William Gilbert (1544–1603) published his book De Magnete, which described his experiments with magnetism and demolished many of the myths that had grown up around the subject. Gilbert described how to magnetise an iron bar: heat the bar, align it to point north–south (septentrio–auster) and hammer it (Figure 1). From his experiments, Gilbert concluded that the Earth has a magnetic field (Figure 2). Read more about the Earth’s field and its possible origins on pages 22–24 in the Crossword notes inside this issue.
Much later, magnetisation was explained in terms of atomic nuclei. Some nuclei, such as iron, have their own magnetic north and south poles. In unmagnetised iron, there are regions (domains) in which the atomic magnets are aligned with each other, but the orientation of each domain is random (Figure 3a). If the iron is heated and hammered, the atomic magnets are disturbed and then settle down to align with any external field, such as that of the Earth (Figure 3b). As with all scientific explanations, the domain theory was developed through a combination of hypothesis (suggestion) and experimental testing (see How science works on pages 29–31). You might like to try thinking of ways in which it could be tested in simple experiments.
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