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Ferromagnetic material is needed for the same reason that transformers working with low frequency alternating current (a.c.) need iron cores. The magnetic field generated by coils in the cooker's hob are supplemented by magnetic fields due to alignment by this field of magnetic domains in the iron. The resulting magnetic flux density may be hundreds of times larger than if there were no iron. Thus as the field changes, because it is a.c. that is passed through the coils, the changes are far greater than if there were no iron, and a much larger voltage is generated. Hence the heating effect is greater, despite the greater resistivity of the iron compared with that of (say) copper or aluminium.