But concerning the Earth, Blackett guessed wrong. Experiments with spinning objects, which by his theory should have produced measurable magnetization, showed none. Later observations also showed that during the last tens of millions of years, the magnetic polarity of the Earth reversed many times, something that Blackett's prediction would never allow.
 | | The core of the Earth |
The Earth's Core The way earthquake waves spread tells us that the Earth has at its center a dense liquid core, of about 1/2 the radius of the Earth--and inside that, a solid inner core. It is widely believed this core is made up of molten iron, perhaps mixed with nickel and sulfur. The density seems appropriate, and iron, which among all elements has the most stable nucleus, is abundant in the universe. It concentrated in the Earth's core because it is heavy--the same reason that, when it is extracted from its ores, it sinks to the bottom of the blast furnace. Energy is the currency with which most processes in nature must be paid for. The Earth's magnetism is no exception, and its energy seemes to come from fluid motions in the Earth's core, from circulating flows that help get rid of heat produced there. In a similar way, our weather is driven by circulating air flows that help cool the ground, where much of sunlight is absorbed. Scientists are still not sure about what provides the heat in the Earth's core. It might come from some of the iron becoming solid and joining the inner core, or perhaps it is generated by radioactivity, like the heat of the Earth's crust. The flows are very slow, and the energy involved is just a tiny part of the total heat energy contained in the core. Scientists are still not sure about what provides the heat in the Earth's core. It might come from some of the iron becoming solid and joining the inner core, or perhaps it is generated by radioactivity, like the heat of the Earth's crust. The flows are very slow, and the energy involved is just a tiny part of the total heat energy contained in the core. So the molten metal is believed to be circulating. By moving through the existing magnetic field, it creates a system of electric currents, spread out through the core, somewhat like Faraday's disk dynamo, discussed earlier. Currents create a magnetic field--a distribution of magnetic forces--and the essence of the self-sustaining dynamo problem is to find solutions such that the resulting magnetic field is also the input field required for generating the current in the first place. Actually, that is only the lowest level of the problem, in which one is free to prescribe the motions. To solve the full problem, we also need information about the heat sources, and these sources must be able to drive motions which also solve the dynamo problem. Such problems are not easy. They involve intricate mathematics and are not yet fully solved. Only the roughest ideas in their solutions can be outlined here. |
