The theories that refer to moving metal within the Earth as the source of the Earth's magnetism directly conflict with a very basic law of electricity and magnetism. It is known as Lenz's Law.
It can be paraphrased as:
If a conductor that is in a complete (closed) circuit moves in a magnetic field, a current will be induced in the conductor. The induced current will have its own magnetic field that is opposite in polarity to the magnetic field that induced the current in the conductor in the first place. This means that it is impossible to have a self-sustaining magnetic field as the basis for the Earth's magnetism.
For those of whose knowledge of electricity and magnetism is weak I offer an explanation based on an item that most people will have some acquantance with - an electric motor.
An ordinary electric motor of the sort used for a car heater's blower is made up of two parts. The outer part is called the stator. The inner part is called the armature. Electricity powers the motor through magnetism. Some electricity is used in windings around the stator to create a fixed magnetic field that envelops the armature. The armature has windings too but these are organised in a special way as a series of loops of a few turns of wire. The ends of these loops are connected to a set of contacts arranged in pairs, one contact of a pair diametrically opposite the other contact on either side of a cylinder around the shaft of the armature. Imagine these as being in positions 9 and 3 on a clock face. Electricity is connected to these contacts by sliding contacts known as brushes. Other loops are connected to contacts in positions 10 and 4, 11 and 5 and so on around the clock face.
When the motor is switched on, a current flows through the stator windings creating a north-south magnetic field that passes through the iron part of the armature. At the same time electricity is connected to one of the loops of wire around the armature. Imagine that the north-south magnetic field of the stator is vertical with north at the top. The current through one of the loops around the armature will create a magnetic field that is more or less aligned east and west. The north part of this magnetic field tries to unite with the the stator's south magnetic field. The south part of the armature's magnetic field tries to unite with the stator's north magnetic field. The effect is to make the armature start to turn. As soon as it does so the loop is disconnected and another loop with an east-west magnetic field is connected in its place via the sliding contacts and the brushes. This second loop's magnetic field works in exactly the same way as the first loop's magnetic field. It makes the armature turn some more.
In a practical motor this process works very quickly making the motor's armature spin at its usual speed. For most people that is all that is wanted. The motor is switched on and it spins. However, checking what is happening in the motor will show an interesting situation. The motor when running will have a current of four amps passing through it. The car's battery supplies 12 volts. The wire in the loops around the armature is fairly thin. It is about the same thickness as 15 amp fuse wire. The resistance of each loop around the armature is around 0.1 of an ohm. If 12 volts was connected across this loop for more than a fraction of a second the wire would melt because the battery would try to drive 120 amps through it. But, as I have said, as soon as a loop has any current through it, the armature rotates a bit, disconnecting the loop.
I have said that the typical heater fan motor uses four amps. Something strange is happening to stop the wire loops around the armature from burning out. This is because the spinning armature also works as a generator. In the example I have offered it generates 11.6 volts in opposition to the 12 volts from the battery. We know this because we know that the motor only uses four amps. As the resistance of the loop windings on the armature is only 0.1 of an ohm, a reverse voltage must be generated to restrict the voltage across the armature loops to 0.4 volts. 0.4 volts/0.1ohms = 4 amps.
Now think of the Earth rotating in a magnetic field. (It doesn't need electricity to make it spin like a motor's armature.) The currents induced in the metal within the Earth will create a magnetic field that is opposite to the magnetic field that induced the currents in the Earth's metal. In other words the magnetism generated within the Earth tends to cancel the magnetism around the Earth.
Therefore no moving metal within the Earth can create the Earth's magnetism.
Wilf James BSc. 05/04/2012