BrightKidz Library
Subjects
The two different ways of making a magnet, side by side Two panels separated by a faint upright line. On the left is a bar magnet drawn as a long rounded rectangle lying on its side, with five small arrows inside it all pointing the same way to the right. Nothing in it is moving; the arrows are simply lined up and staying there. On the right, a faint dashed circle stands for the inside of a planet, and within it four curved arrows chase each other round in a broken ring with clear gaps between them, showing liquid metal circulating. A single short arrow at the very top points up and away from the dashed circle, standing for the magnetic field that the circulation sends out. The left magnet works by things lining up and holding still. The right one works only while things keep moving.

The Earth Is Too Hot to Be a Magnet

About 8 minutes

Your needle floated, turned, and stopped facing North. The reason given was that the Earth is like a giant magnet.

Like which magnet, though? Because if the Earth were a giant version of the one on your fridge, it could not possibly work — and the reason it could not is something you can check on a cooker.

What a fridge magnet is made of

Inside a piece of iron are millions of tiny patches, and every patch is already a small magnet. In a plain nail those patches point in all directions, so their pulls cancel and the nail does nothing. Stroke it with a magnet and they swing into line, and now their pulls add up.

That is what you did to your needle. You did not put magnetism into it. You tidied up magnetism that was already there.

And this is why the instructions were so fussy about stroking the same way every time. Each stroke turns the patches a little further in one direction. Stroke back the other way and you turn them straight back, which is exactly why rubbing to and fro undoes your own work.

Heat undoes it too

Now the important part. Those patches only hold their line while the metal is reasonably cool.

Heat iron past about 770 degrees Celsius and the patches lose their grip on each other and scatter. The magnetism goes, all at once. Let the iron cool again and it does not come back, because the patches are pointing every which way once more.

That is a real number and it is not especially high. Iron glows before it reaches it.

The Earth's core is nowhere near cool

The middle of the Earth is iron. It is also about 5,000 degrees Celsius.

That is not a little over the limit. It is more than six times past the temperature at which iron gives up being magnetic altogether. Nothing down there could be holding a line, because everything down there is far too hot to hold anything.

So the Earth is definitely not a giant fridge magnet. And yet your needle still turned.

The other way to make a magnet

There is a second way, and it has nothing to do with lining anything up.

Moving electricity makes a magnetic field. Not stored, not lined up — moving. Send electricity along a wire and a magnetic field appears around it, and the moment the electricity stops, the field vanishes. Nothing is left behind, because nothing was ever tidied into place.

The Earth's outer core is liquid iron, and liquid iron carries electricity extremely well. That liquid is not sitting still. It is churning, in enormous slow currents, heated from below and spun by the planet's own turning. Moving metal, carrying electricity, in a mass three thousand kilometres down.

That churn is the magnet. Not the iron — the motion of the iron.

Two explanations, and each one predicts different behaviour. Sort each observation under the kind of magnet it fits.

  • It stops working if you heat it enough
  • It needs something to keep moving, or it fades
  • Its poles stay where they are, year after year
  • Its poles wander, and can even swap ends
  • You can make one by stroking a needle
  • It survives at temperatures that scatter every patch

The evidence is in the wandering

Look at the last two on the right-hand side, because those are not guesses. They are measurements, and they rule the fridge-magnet answer out on their own.

Magnetic north moves. It is not a fixed spot. It has been drifting for as long as anyone has measured it, and lately it has been travelling tens of kilometres every year — it has crossed out of the Canadian Arctic and headed towards Siberia. Ships and aircraft use charts that have to be updated because of it.

A bar magnet does not do that. Its poles are where its lined-up patches say they are, and they stay put. A swirling ocean of liquid metal, on the other hand, would be expected to shift, because the swirl itself shifts.

And it has flipped. Rock that cooled long ago locked in the direction of the field at the time, like a photograph of a compass. Read those rocks in order and the field turns out to have swapped ends many times, north becoming south and back again. The last full flip was around 780,000 years ago.

No lined-up magnet has ever reversed itself. A churning fluid could, and the record says it did.

If the Earth's outer core cooled down and stopped churning, but stayed made of iron, what would happen to your compass?

Go back and look at your needle floating in its bowl. It is being turned by something 3,000 kilometres beneath the floor, made of metal too hot to be a magnet, held up entirely by the fact that it will not stop moving.