I came across a few surprises after reading North Pole, South Pole by Gillian Turner. The major surprise was that the Earth actually has at least FOUR magnetic poles, not two. There may be even more poles, but if there are, they manifest too weakly or indistinctly from the other four poles to be noticeable. The four poles of course come in two pairs, North and South.
The current orthodox view is that the Earth’s geo-magnetism is caused by circulating electrical currents beneath its surface. It was Ampere who first discovered that a loop of electric current produced a magnetic field. Others eventually thought of applying his discovery to Earth’s magnetic field (or fields).
Geo-magnetism does indeed seem to advertise the presence of at least one powerful electric current inside the Earth. The Earth, after all, is a spinning ball full of countless electrons, so yeah, the idea that its rotation could give rise to a gigantic electric-dynamo inside is not a stretch. Plus, the magnetism produced by the Earth is typical of that produced by a loop-current– a pervading field of magnetism which is strongest at the poles and weakest at the middle.
In 1692, Halley had already made a pretty good guess as to the reason behind the extra set of magnetic poles. He reckoned that there must be TWO electrical currents circulating inside the Earth, and that they are not aligned. He figured also that there must be an insulating layer between them (my guess is that he figured that, without the inbetween layer, the two currents would probably merge or interfere with each other). Halley considered that the two conducting materials were solid, and the inbetween layer was liquid. Each of the two conducting spheres would create its own set of magnetic poles. Halley also said that there could be several other spheres of current creating several other sets of magnetic poles.
But here’s something that I find bizarre and suspicious… the North and South poles for each pair of magnetic poles are NOT truly opposite from each other as is normally the case– as far as I know– with every magnet every made or discovered. The Earth’s magnetic North is NOT directly opposite and inline with its magnetic South (that is, the two poles are not 180 degrees away from each other). And all the poles are merely in the general direction of the ends of Earth’s rotational axis, but never right on top of them. Strange…
Also, Halley thought the spheres of electricity inside the Earth rotated at different speeds, so that periodically– say every so-many centuries– they would overlap.
Christopher Hansteen, working in the early 1800s, agreed with Halley that there are at least four, not two, magnetic poles. He also concurred that none of the poles actually occur at the axis-poles of the rotating sphere of the Earth. Furthermore, Hansteen noticed that the locations of the poles are not stationary… they have been moving around the globe for probably millennia.
Scientists soon enough began using information concerning the Earth’s magnetic field to make assumptions about the composition of the Earth’s innards. As Hansteen remarked in 1819, “the Earth speaks of its internal workings through the silent voice of the magnetic needle.”
In 1896, Emil Wiechert speculated that the Earth must have a very heavy Core. To get his theory to work-out right, the Core had to be much heavier than the average weight of the Earth (5500 kilograms per cubic meter), and the only thing THAT heavy would be metal; Wiechert proposed Iron.
The inside of the Earth is supposed to be very, very hot. So it would be understandable to picture the supposed iron as liquidy, even gaseous– however, the pressures down that far are also amazingly intense, so the idea of a Core of gas probably won’t fly… but I have heard some back and forth as to how liquidy versus solidy the depths of the Earth are.
Regardless of the precise nature of the Earth’s core, the idea that there are different layers –possibly even layers of different matter-states– lends support to the idea that these layers would be behaving differently– including rotating at different speeds. It’s also been suggested, quite plausibly, that the gravitational pull of the moon would exercise different “drag” effects on the different levels– thus exacerbating or perhaps even causing any differential in rotational speeds.
Personally, I’m a little concerned that the picture of the Earth’s internal composition and electrical characteristics is just a little TOO nice and tidy… For one thing, surface evidence indicates that, beneath the surface, there’s a hell of a lot of circulation of hot fluids going on (magma, for instance) that is NOT primarily in the direction of the Earth’s rotation but occurs in several distinct, rotating cells– the rotation being up and down… that is, the hot liquid is moving up toward the surface on one side of a cell, and then back down toward the Earth’s core on its other side. Where two side-by-side cells are both moving up (locked together like gears in a machine), the current is moving UP and there are steady eruptions and lava flows (this is thought to be occurring at the ocean floor). Where the currents are moving down, there is “subduction”– when one surface-plate of the Earth is drawn beneath another, overlapping one.