I wanted to show you one of my favourite props from last

year's Christmas Lectures, which is a Mobius strip steel

track, covered in maybe 2,000 or 3,000 of these super strong

neodymium magnets.

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So what I want to show you is how this track is going to

interact with one of these.

This is a high temperature superconductor, made of

yttrium barium copper oxide.

It's a sort of ceramic material.

And a superconductor is something that, when you cool

it down, in this case with liquid nitrogen, it loses all

its electrical resistance.

So this leads to some interesting behaviours, which

I'm going to show you.

And to do that I'm going to need to hang this thing from

the ceiling, which I'll do now.

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Oh!

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So we have one suspended, super strong, neodymium

magnetic Mobius strip track.

What we need now is some liquid nitrogen.

So we're down in one of the labs at the RI.

And this is where the liquid nitrogen is stored.

Right.

Got what we need.

The superconductor is now cooling down, which will take

a minute or so.

It needs to be cooled down in situ, because, one, what I'm

trying to get to is to have a superconductor kind of locked

into this position.

So I need to cool it down in situ above the track.

I'm cooling it down past it's critical temperature, the

temperature at which it will become a superconductor, and

its electrical resistance will completely disappear, which is

about minus 180 degrees.

And the burger tray is there to maintain the spacing

between the superconductor and the track.

I mentioned that we're trying to lock it

into a specific position.

So that is the position that we want it to take up.

I'm just going to slide this off.

This is the best way I've found of doing this.

If you ever hold it so quickly enough you can see that it's

actually levitating on the track.

Actually, you might be able to see a gap underneath it there,

sliding back and forth, levitating.

Oh!

OK.

So unfortunately, it warms up quite quickly.

Obviously, the effect only works as long as it is at its

superconducting temperature.

So what's actually going on there, a good way to start to

understand that is to think about how magnets interact

with ordinary conductors, like this copper tube.

So if I just drop this nut through, it falls all the way

through very quickly, as you'd expect.

If I drop a strong magnet through instead, it takes

quite a long time.

So the reason that's happening is as the magnet moves through

the copper, it induces electric

currents in the copper.

Those electric currents, themselves, have their own

magnetic field.

And that magnetic field will always arrange itself so as to

resist the motion that's creating it.

This is the fundamental principle of electromagnetism,

the moving magnets.

And the conductors are moving conductors near magnets,

creating electric currents.

Now, because this is not a superconductor, because it has

electrical resistance, the currents that are created as

the magnet falls through will always tend to die away.

So if the magnet were, for instance, to be stopped by the

electric currents, those currents would immediately die

away, and the magnet would start falling again.

So what actually happens is it just falls

slowly through the tube.

If, however, this were a superconductor those currents

would not die away.

As soon as you induce them by introducing the magnets into

the tube, those currents would remain and would keep going

and would keep going even after the magnet has stopped.

So if this were a superconductor, the magnets

would basically be locked in the pipe.

They would be effectively levitating in the pipe

indefinitely.

So that's part of the explanation.

We are sort of halfway there.

And we'll get onto the track now.

And to do that, we saw, when I was demonstrating the

superconductor on the track down here, that the effect

only lasted a few seconds.

It warmed up too quickly.

It warmed above its critical temperature and stopped

levitating.

The solution we've decided on for the Christmas Lectures

dossier was to make it a little train

stroke, boat thing.

So we've got a piece of the superconductor embedded in

polystyrene with its own little

reservoir for liquid nitrogen.

So let's cool this down.

Again, it's not a burger tray this time.

It's a slightly different dish.

Gently pick it up.

And I think--

So now I can send her out.

Come all the way back.

Come back on top.

This is why we made it like a Mobius strip, so that it can

go and come back on the opposite side it went around.

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So if I stop it there.

So it's kind of locked into position.

So it's not just repelling the magnets and hovering.

I can sort of pick it up and then stick it underneath as

well, and it will hang there.

So as I mentioned with the copper, when you induce

electric currents in conductors with a moving

magnetic field or changing magnetic flux, the electric

currents that you create will always have their own magnetic

field that resists that change, that

resists that motion.

So when it's hovering on top of here, gravity is trying to

pull it towards the track.

That small motion toward sets up the electric currents in

the superconductor, which have a magnetic field that resist

that motion.

But likewise, when it comes around and is now on the other

side, now gravity's trying to pull it away.

So the act of trying to fall away from the track sets up a

different sets of currents with a different magnetic

field, which this time are attracted to the track.

So whichever way you try and move it, whether you're trying

to move it off to the side, up and down, towards the track,

away from the track, the superconductor will resist

that motion, effectively locking itself into position

relative to the track.

So the only direction it can move is the direction which

the magnetic field doesn't change, i.e.

along the track.

So basically, the superconductor can be any kind

of magnet it wants to be, or any kind of magnet it needs to

be at that moment to keep itself in the right position

relative to the track.

So this material was a real breakthrough and possibly the

first step on the way to what might be the holy grail of

superconductor research, which would be a superconductor that

would operate at room temperature or close to room

temperature.

At which point, we might be able to use them to replace

all of the conventional conductors, like copper and

things, that we use in all our electrical applications today.

And all those materials, when they conduct electricity they

do it inefficiently to some extent and waste energy.

If we could superconduct in those applications, then that

would really change the world.

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