Professor Eric Laithwaite: Magnetic River 1975
permanent magnets are difficult things to understand in fact if we're absolutely honest with ourselves we don't understand them you see either end of a bar magnet will attract a piece of steel but it doesn't follow that because it's painted ready to magnet because wood it won't pick up that piece of steel but this one will so this one's the magnet but the only real test between two magnets is whether or not in addition to attracting each other they will also repel when reversed now the idea of repulsion gives you the impression that perhaps you could float one of the magnets over the other like this now I can feel the repulsion now watch what happens when I let go ah you see he won't stay there you get repulsive force but we say it's not stable now people have proposed this idea for suspending railway carriages over the track because it uses no power but then you have to do something about stopping it falling off sideways now we can make ring magnets like these in this case the whole of one side is a North Pole and the whole of the other is a South so if we get two of them we can try and float one over the other and again you see you get the lift but we can't get the stability unless we put in a glass tube and you get floating or it's not just marvelous people are suggested you could use this for floating railway carriages but you see the clearance there perhaps isn't big enough so what we'd really like to have is some more...
magnets to make it stronger that's going in roughly the right direction are now we're getting a lot of clearance but of course these magnets would be on the track and this would be the vehicle so this is going to be very expensive you want hundreds of miles of this so what we'd really like to do would be to put a single magnet on the track and add more magnets on the vehicle two magnets three magnets the gap instead of getting bigger now is getting smaller so this is a situation which gets worse as it gets bigger so it is no good for railway vehicles now there's another kind of magnet which doesn't involve the use of rings or anything like that instead a coil of wire we're going to take an iron bar and push it through the center of the coil and then pass an electric current through the coil this meter will tell you how much current is passing you'd connect that one very I'll connect how you connect the switch all right now all we need is a battery to connect to here and we should be able to make a magnet out of that bar of Steel which of the Lord is not magnetized if you give me the board Donny we shall be able to see whether it behaves truly in the manner of a magnet because here is a magnet at the moment this is just a barn of iron so the magnet will attract either end now let's see if we can make it repel by switching on yet current that end will now attract and that end repel so now we've made an electromagnet but will it work...
without the iron core we just use the coil alone but it certainly attracts and the turn it round and it just repels so it is still a magnet even though it has no iron in it at all but of course if we put the iron core back then the attraction repulsion is far greater so we make a better magnet now let's see how much weight we can lift with this magnet very approximately the iron bar I'll see if I can pick up this heavy iron bar and we can just it's not actually jumping up to meet it but having made contact I can just lift it but there's more magic in an electromagnet if instead of feeding it from a battery we feed it from alternating current look at this oscilloscope you see a spot rising and falling that spot is really measuring the amount of current that flows in a wire when it's above the middle line the current flow is one way and when it's below it flows another now I slowed this down to show you what is really happening but in your house you have an electricity supply which goes much faster up and down even than that faster even than that it goes up and down 50 times a second now let's have a look at an electromagnet over here that uses that kind of supply and this is a coil of wire with a long iron core this is a ring of aluminium aluminium you know is a non-
magneticsubstance we can't pick it up with a bar magnet as you can for example a bunch of keys so we've got this non-
magneticring over this iron core and switch on you see that...
you get the ring thrown high into the air if without switching off I put it back and it floats but notice it never floats level and in the center it always has to lean on the pillar just like the permanent magnets did if I put a thinner ring on doesn't float quite so high finish still lot lower down and if we go to the ultimate and cut a ring from kitchen foil is also aluminium that doesn't float at all but unlike our permanent magnets if we now add some more rings we shall begin to see the whole lot lift remember with the permanent magnets the more magnets we put on the top and the smaller is the lift and in this case the more the rings the higher the lift put the thicker one on thicker still and up it comes right to the top now what is the secret of this amazing device this is the giveaway this is a copper cylinder that is free to spin if I hold the copper cylinder alongside there's nothing much going on but if I take this ring which wants to float up there and I force it down alongside the copper then the copper begins to spin and if I put it on the other side it spins the other way it is as if there was an upward sweeping something which is causing the copper to spin and we call that up with sweeping something a traveling
magneticfield now this is a row of electromagnets arranged deliberately to produce one of these traveling
magneticfields so when I switch it on put our copper cylinder over it and there you see the same effect as if something was sweeping...
underneath it let me put a bit of aluminium on it does push it along but not very fast let's try a bigger piece that's better bigger still better still you see the bigger you go and there better it gets this of course we call a linear motor now suppose we increase the current would you mind increasing the current Barry and we can now hold a sheet of aluminium which is not only trying to be pushed along but it's also lifting above the surface it's much more alike what you would expect if you threw a piece of wood into a flowing
riverit would float and move along so this then is the beginnings of a
Riverbut it hasn't got any banks because you're right take my phone away from me it'll pop the sign if I take it too far towards you off the other side but if only we could stabilize it then we should have something that looked exactly like a
rivernow how does all this work could I have that all he's right this is a mechanical model of what you've just seen instead of a row of coils we've got a row of locks we're going to feed them with alternating current so that each one can rise and fall and when we turn the whole thing we get the impression of something traveling along but I want you to notice that nothing actually travels along because each rod only moves up and down the thing that makes it real is when we put something into one of the troughs of the wave then something real travels along connecting on the other work if you...
watch an individual rod such as this one you'll see that it's only going up and down there is no horizontal movement at all only the ball is a real thing moving side to side it's exactly the same with this
RiverI can switch it on and there is nothing above there nothing moving is on telly until I put in the aluminium and light putting the ball into the rod so this is our first step towards a
Riverthis is a single coil and the plain sheet of aluminium so it will behave rather like our jumping ring switch on and it goes try and put it back it floats it has a really good try and floating but it doesn't make it but if we place it not in the middle of the coil say it like that and it doesn't jump straight up that breadth of this side I put it towards link so on let us use this spinning cylinder again to see what's going on switch on put the cylinder in the center nothing happens introduce the plate and it spins introduce the plant from the other side spins the other way notice which way it's trying to throw the plate out so to try to do this is like trying to balance a pencil on each point you just won't succeed but this idea of producing traveling fields like this it's given us an idea if we could only produce inward traveling fields without the plate there then we might have a chance of holding the plate in the middle so let's try and do that instead of a single coil I'm going to use two coils one inside the other...
this is a coil and that this is a coil and these three rings are steel rings there to help to strengthen the
magneticfield now we found that we can produce this inward travelling field if we make the current in this coil go that way around at the same time as the current in this one is going the other way around so when we put both coils on together we can perhaps detect with the copper cylinder a very slight tendency to produce inward travelling fields that is the bottom of the cylinder moving towards the center that's what we're looking for the
magneticfield takes no notice of fishes of wood or cardboard or paper so I can pass a piece of wood underneath without affecting the floating disk in any way now I want you to imagine that we've been able to take this solid structure put it under a steamroller and roll it out flat really flatten it in this direction let's try and imagine what that would look like well here's our aluminium plate and we'll switch it on there is our aluminium suspended but not yet movable I can push it by hand it's as if we had a
Riverwithout any flow it's quite stable but it needs something to push it so just to demonstrate that I'm going to put on a little air screw on it and when I switch this on you get your propulsion there was a time when we thought we should have to put a linear motor up the center between the coils to get propulsion as well as lift and guidance but a few months later we discovered...
that all it was necessary was to break up these two coils into a set of smaller coils and we could get the lift propulsion and guidance from one of the same set of coils now here is one row of coils and the other one at the other side but this time the coils have been divided up into groups so that we can produce a traveling field as well as give lift and guidance the thing we're going to propel in the