You might think the whole effect is something to do with the echo in this hall.

That's what at first sight it seems to be.

But it's a thing named after Edwin Hall, who invented the idea in 18 79.

But you could actually get some funny effects of a magnetic field near a wire.

So let me show you about that before I talk about the whole effect.

Let me explain something else which is called the Lorentz Force.

And in this apparatus, I have electric charges coming out of here on they are knives, the gas and make it glow green.

So you probably seen something like this before.

Now this is a magnet, a really strong magnet.

So this is put in little I know 36 millimeter camera so that he doesn't clang again to other things.

And if I put that close to this, you can see that the beam of electrons coming out of there giving that green glow is deflected.

It goes up.

If it's near this pole and it goes down if I turn it around for the other poll, so the effect of a magnetic field.

If the electrons are running this way is to bend it up puts.

Now, I could do the same experiment, this one and I put a sheet of metal.

This is aluminium, and I'm gonna put this sheet of metal in the way of the magnet.

Have you got the beam still there, Brady?

Because I'm gonna put this close to the metal and I'm not cheating.

You take the metal way.

It's still there.

Put the made metal there.

It's still bending.

The magnetic field goes through the metal.

Any metal, when it's behaving normally will allow magnetic fields to penetrate it.

That the question posed by Edwin Hall Waas What happens if you have an electrical current down a wire, as in these wires?

But I've blown it up so you can see it quite clearly on I put a magnetic field near it.

Now then you've seen already that if there is a electric charge running along in this direction, a magnetic field will make it bend.

So the magnetic charge is going along here and it's going to go on.

Bend on totally hits a wall.

So if I now think about a wire such as this without a magnetic field on.

Let's pretend this is a charge carrier, so you can really see what's going on.

I mean, this is not really to scale.

Here's a bit of the wire on the charge goes through here flowing down the wire and the current is just the number of charges moving down the wire per unit time.

So if I put a magnetic field down here on the charges air coming along, as you've seen from this experiment, it's going to bend up and come to rest on the edge of the wire.

And if I do this straight off on, then send another one down.

If two will do this when I got negative charges there, so I'll cancel him out with positive charges.

Over here on down, we'll come.

Another one on it will suddenly say, Oh, I want to go here and there will be a positive charge induced on the other side.

So now when another charge comes along, it wants to be attracted this way because it wants to go near the positive charge.

But on the other hand, there's a magnetic field which wants to sleep it that way on the balance of these two means that it doesn't choose to go that way or that way, just carries on down the middle, not being attracted to either side.

And as a result, after these charges have built up on one side on the other, it gets to the steady state so more and more charges can come through Justus.

If this wasn't there, so there's no reason to expect that there's any effect at all.

The current carries on flowing just like water going down a tube.

You don't know that there's anything happening or either side.

The upshot of all this is the magnetic field does not affect the current going through the wire.

But you now get between the negative charge and a positive charge, a difference in electric potential, which you can measure using a suitable meter.

So the idea is this.

You send a certain current through here, and you measure this voltage from one side to the other.

This is not the same Mazzone's law, where it's the voltage down the wire.

It's the voltage across the wire that you're measuring.

It's quite different on this is the idea that hole came up with.

So you can measure this voltage divided by this current, which has a unit of resistance because it's a voltage times a current.

But it's nothing to do with those law.

This is the whole effect, a voltage this way, with a current flowing that way, you probably want to know Brady what the use of this is, Well, I could get some very clever modern electronics if I can turn this thing on and you know how I am Brady.

I'm a theoretician.

I touched pieces of apparatus and they break down.

So this might take a little Walter toe workout right at the end.

There there's a tiny little square about a millimeter square.

Orbit bigger on wires go into an out of it, and that is the whole probe.

This small little square is the analog of this big one on.

There are currents going into it on.

There is a voltage across it.

So if I put this in a magnetic field and I put that in there and you suddenly get oh, it's gone off the screen, you could measure this in Tesler.

That's 88 point hunt 100 Milli Tesler in there and you get in the middle.

It goes off the scale so you can actually wander around the room with this on Measure the magnetic field in any particular direction.

So if I have the square like that, it's a magnetic field coming in perpendicular that you're measuring.

If I twist it around and get a different magnetic field, so you might want to invent things using this on one of the things which is used using this effect is a burglar alarm.

You put a tiny little magnet in your door, apparently on the whole probe in the jam, and set it all up with electronics on.

As soon as the burglar comes in the door, this moves away from the magnet and there's a huge change in the magnetic field and you get a nice current A.