Magnetism and electromagnetism · System
The Earth is a magnet
A compass works because it is one magnet finding another one, and the other one is the planet you are standing on.
Start here
Nothing is near it. It still finds north.
Put a compass on an empty table in the middle of an empty field. There is no magnet within a mile of it, and no metal. Spin it, and it settles pointing the same way every single time. Carry it a hundred kilometres and it still settles the same way.
What is the needle finding?
The planet. The Earth has a magnetic field that reaches everywhere on its surface and out into space, and a compass needle is simply a small magnet free to turn, so it does what any magnet in a field does and lines up with it. Nothing has to be nearby, because the field is not made nearby — it is made in the Earth's core, thousands of kilometres down.
A compass needle is a small magnet, balanced so it can turn. A magnet turns until it lies along a magnetic field. Since the needle settles the same way everywhere on Earth, there must be a magnetic field covering the whole planet — and there is. The Earth behaves as though a huge bar magnet were buried inside it, tilted a little from the axis it spins on.
There is no actual bar of iron down there. The Earth's core is far too hot for that: above a few hundred degrees a magnet loses its magnetism altogether. What is down there is an ocean of liquid iron, and its slow churning carries electric currents, and moving charge makes a magnetic field. The buried-bar-magnet picture is a model of the field's shape, not a description of the rock.
Now the part that catches people out. The needle's north-seeking end points towards the Arctic. Unlike poles attract. So the magnetic pole up there must be a south pole, and the Earth's field runs from the far south, round through space, and back into the north. The naming is historical: the ends of the needle were named for where they pointed, centuries before anybody knew why.
The magnetic pole is also not in the same place as the geographic one, and it wanders — by tens of kilometres in a year. The angle between the way the needle points and true north is called the declination, it is different in different countries, and any map used for serious navigation prints the local value and the date it was measured.
At the bench · a compass free to swing in any direction
Take the same compass somewhere else.
Change a control to begin
This compass is hung at its centre so it can tip as well as turn.
Commit first. A walker carries an ordinary compass to a point directly above the Earth's magnetic pole in the Arctic. What does it do?
52° north — southern England
On the bench beside it
How it is mounted
Which way it turns
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Angle of dip
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Sideways pull to work with
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where 100 is the strongest here
Can you navigate by it
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The figure
Three norths, and only one of them is a magnetic north
True north
The end of the spin axis
Where the lines of longitude meet. It is a fact about how the Earth turns and has nothing to do with magnetism. It does not move.
Magnetic north
Where the needle points
Hundreds of kilometres away from true north, and moving year by year. In magnetic terms this place is a south pole, because the needle's north end is attracted to it.
Grid north
Straight up the map
The direction the squares on a printed map run. Flattening a curved planet onto paper bends things slightly, so this is a third answer again.
The angle between the needle and true north is the declination. Walking maps print it in the corner with the year it was measured and how fast it is changing, because a bearing taken from an old map and followed with a modern compass will put you in the wrong valley.
Key fact
The Earth has a magnetic field shaped as though a bar magnet were buried inside it, made by moving liquid iron in the core rather than by any solid magnet. A compass needle is a small magnet lining up with that field. The magnetic pole in the Arctic is a magnetic south pole, which is why the needle's north-seeking end turns towards it — and it sits some way from true north, and moves.
Think again
“The compass points north, so the Earth's North Pole is a magnetic north pole.”
It cannot be. Unlike poles attract, and it is the north-seeking end of the needle that swings towards the Arctic — so whatever is up there has to be magnetically a south pole. The wording is the leftover of an old decision: the ends of a needle were named for the direction they pointed long before anybody knew a field was involved, and by the time the physics was understood the names were on every chart in Europe and were not going to be changed. Read “north pole of a magnet” as “the end that seeks north” and the contradiction disappears.
“A compass points at the North Pole.”
It points along the Earth's field where you are standing, which is a different thing. In Britain that happens to be within a degree or so of true north at the moment, which makes the two easy to confuse; in parts of Canada and New Zealand the gap is more than twenty degrees, and following a compass bearing there without correcting for it puts you kilometres off over a day's walk. The needle is not aiming at a place. It is lying along a line, and the line only has to reach the pole eventually.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Name the pole
The north-seeking end of a compass needle turns towards the Arctic. What does that make the Earth’s magnetic pole in the Arctic?
Rung 2 · Use the model
A walker in Britain takes a bearing from a map printed forty years ago and follows it exactly with a modern compass. Why might they end up off course?
Rung 3 · Explain
Explain why a compass works at all, starting from what the needle is and ending with what it is lining up with.
Rung 4 · Take it somewhere new
Aircraft flying polar routes have used the same magnetic compass as everyone else, but crews are trained not to rely on it above a certain latitude. Explain what goes wrong with a compass close to the magnetic pole, and suggest what a crew could use instead.
Key note
A compass needle is a magnet, and a magnet lines up with whatever field it is in. The needle settles everywhere on Earth because the whole planet has a field, shaped like the field of a bar magnet buried inside it and tilted from the spin axis. Nothing solid down there is magnetised — the core is far too hot — and the field comes instead from currents carried by churning liquid iron. Because unlike poles attract, the magnetic pole in the Arctic is a magnetic south pole. It does not sit at true north and it moves, so a bearing worth trusting comes with a declination and a date.
Going further
The field has swapped ends many times. When lava cools, the iron minerals in it set with the field of the moment frozen into them, so a stack of old lava flows is a stack of dated compasses — and reading down a stack shows the direction flipping over and over, hundreds of times, at intervals of a few hundred thousand years. The last reversal was about 780,000 years ago. Nobody can predict the next one, and during a reversal the field does not vanish so much as become tangled, with several poles at once for a few thousand years.
Plenty of animals read the field. Robins, sea turtles and salmon all navigate by it, and some bacteria grow a chain of tiny magnetic crystals inside themselves that swings them into line like a compass needle, so that swimming forwards takes them down into the mud they need. It is worth being careful with the claim, though: exactly which organ does the sensing is still argued about for most of these species, and it is one of the genuinely unsettled questions in biology rather than a finished piece of textbook science.
Before this lesson
Connects to
At GCSE this becomes
- The Earth's field as evidence for a dynamo in the core, and magnetic striping on the sea floor as evidence for plate tectonics.
Where to next
Ask Mr Badmus AI
Confused about why magnetic north is a south pole?
The bench is a teaching model. The Earth's field is treated as that of a single bar magnet at the centre, lined up with the spin axis, which is the standard first model: the angle of dip is then given by a fixed relationship between dip and latitude, and the sideways pull is what is left of the field once the tipping is taken out. A real field is not that tidy. The magnetic axis is tilted about eleven degrees from the spin axis, so magnetic latitude and geographic latitude are not the same and dip measured in the field differs from the figure here by several degrees in most places; the field also has large local variations from the rocks below, and the poles move by tens of kilometres a year. Whether a steel object beside the compass wins is decided here by a single latitude, which is a stand-in for a comparison that really depends on how big the steel is and how close it sits. No value in tesla is given, because the unit is beyond this stage. Angles are rounded to the nearest degree and the sideways pull is a relative figure with the strongest value on this model set to 100.
Lesson content © MrBadmusAI.