The picture everyone has seen
You have probably seen the video: a small magnet rests on a little ceramic disk, someone pours in liquid nitrogen, white mist boils off, and the magnet rises and hangs in the air, perfectly still, as if gravity forgot about it. The disk is a superconductor, and what holds the magnet aloft is a phenomenon called the Meissner effect. (The Meissner effect provides the upward push; in this liquid-nitrogen ceramic-disk demo the magnet is locked so perfectly still by an extra trick called flux pinning, which we meet in a later guide — pure Meissner repulsion alone would lift the magnet but not hold it steady.) To understand it, we first have to clear up a common and tempting mistake.
The tempting mistake is to think: 'It has zero resistance, so the levitation is just a side effect of that.' It is true that zero resistance is part of the story. But the Meissner effect is a separate, deeper property — and in fact it is the better test of whether something is truly a superconductor. Let us see why.
Perfect conductor versus superconductor
Imagine a magical metal that simply has no resistance, but is otherwise an ordinary metal — call it a perfect conductor. There is a rule of physics that any change in the magnetic field through such a metal stirs up swirling currents that fight the change. With no resistance, those swirling currents never fade, so they lock in whatever magnetic field happened to be passing through at the moment the metal became perfect. The field gets frozen in place.
Here is the crucial difference. Take a real superconductor, sit it in a magnetic field while it is still warm, then cool it past its critical temperature. A mere perfect conductor would trap the field that was already inside it. But the real superconductor does something more dramatic: as it crosses into the superconducting state, it pushes the field completely out of its interior, expelling it whether the field was there first or not. That active expulsion is the Meissner effect, and a frozen-in field could never do it.
How a magnet floats on nothing
Now the levitation makes sense. The superconductor refuses to let the magnet's field inside. To keep its interior field-free, it summons up screening currents on its own surface — currents that flow with no resistance and so never tire. Those currents create their own magnetic field that exactly cancels the magnet's field inside the material. Outside, that surface field acts like a mirror image of the magnet, and a magnet's mirror image pushes back on it. The push is upward, it balances gravity, and the magnet hangs.
Because the superconductor reacts by opposing any magnetic field pushed at it, physicists say it is a perfect diamagnet. A diamagnet is any material that weakly repels a magnetic field by setting up an opposing field of its own; water and your own fingertips do it ever so faintly. A superconductor does it perfectly and completely — it is, in this sense, the strongest diamagnet there is.
- A magnet's field tries to thread through the cold superconductor.
- The superconductor sets up tireless surface currents to cancel that field inside.
- Those currents make an outside field that mirrors and repels the magnet.
- The upward repulsion balances gravity, and the magnet floats.
How much field is too much
Pushing a field out costs the superconductor energy, and it can only afford so much. If you ramp up the magnetic field strong enough, there comes a point where it is cheaper for the material to give up and let the field flood in — at which moment superconductivity collapses and the metal goes ordinary again. That breaking-point field strength is called the critical field. Like the critical temperature, it is a limit: too hot, or too strong a field, and the magic switches off.
There is a twist worth flagging now. For the simplest superconductors, that wall is sharp: below the critical field they expel the field completely, above it they give up entirely. But a second family of superconductors handles a strong field far more cleverly, letting it leak in a little at a time instead of surrendering all at once. That clever compromise is what makes powerful superconducting magnets possible, and it is the whole subject of a later guide.
A hint of the quantum underneath
There is one more clue worth keeping in your pocket. In some superconductors a magnetic field can sneak through a ring or a thin tube, but never in any old amount — it can only pass in fixed, identical packets, like coins of a single denomination. This is called flux quantization: the amount of magnetic field threading a superconducting loop always comes in whole multiples of one tiny basic unit, never a fraction in between.
Why would magnetism come in whole coins? Because deep down a superconductor is a single quantum thing spread across the whole sample — a state physicists call macroscopic quantum coherence, meaning the strange counting rules of the quantum world, usually hidden inside single atoms, here show up at a size you can hold in your hand. Flux quantization is the smoking gun for that. We will meet the underlying pairing of electrons that makes it possible in the next guide.