Professor Eric Laithwaite: The Circle of Magnetism - 1968

Oh well, you can't win them all, can you? What am I an electrical engineering

professor

playing ping pong balls? It's not just that I think science should be fun. I have a serious purpose in mind and it is not. The part of the shoot that interests me is how these balls remain suspended in the water jets which really fascinates me. You could ask, of course, what an electrical engineer is doing duplicating in Hydraulics and the answer is that I can see what is really happening. my own themes are electricity and

magnetism

and both are invisible. I'm trying to make a model of what could actually happen in a magnetic circuit that behaves exactly the same way as this water circuit, in other words I'm trying to make the invisible visible now the thing about analogs is that You need to know to what extent they can be useful and just when you have to take them down.

I want you to see this as a circuit, watch when the water hits the throwing ball. back to the channel and then returns through pipes back to the pump. Now, of course, we can make everything neater by enclosing everything in pipes like this in this circuit, the driving member is here, the pump, this is a valve or switch when you open the valve you will see a plunger go up in this pipe here this is registering the flow rate of water here is a manometer that records the pressure difference between two points in the circuit the manometer is connected here and here to measure the pressure in this piece of pipe and this water circuit has been designed to be analogous to an electrical circuit like this instead of a pump there is a battery instead of a valve a switch and instead of a water flow meter an electric flow meter or a meter, this voltmeter can be connected to that point or to this point and by subtracting the two readings we can measure the electrical pressure drop across this part of the coil, so I'm using something I know to help me explain something I know less about now, the problem with analogs is that none of them It's completely true, so let's see how far we can take this electric water analogue before it breaks down, first of all if I replace this rigid tube with a flexible hose and then when I turn. the pressure on it would leak out like a garden hose does and it would try to turn into a

circle

.

Now the same thing will happen with the electric

circle

. Suppose you replaced this coil with a more flexible one. What do you think would happen? I'm going to use a loop of aluminum foil. Regular kitchen paper will do, but the thinner the better, when I turn on the power you will see it curve like the water pipe. I must not leave the power on for too long each time, otherwise. I'm going to burn the tape. It takes about 20 amps to do this. Now I'm going to try something different. I'm going to hold a bar magnet next to it and this time when I turn it on something very different happens and of course there is no equivalent. of this in the water circuit unless I can get a water diver and his hazelnut twig, but you know you always get into trouble when you try to explain the behavior of permanent magnets let me show you what I mean.

I use two bar magnets and first of all I place them with opposite equal poles, so of course they repel each other. If I put some steel now on the pole of that magnet, I don't change the polarity just by doing that if that was a North Pole before then, the outside of the steel will still be a North Pole and therefore it should still repel to the other magnet, shouldn't it be surprising? Now in that condition both magnets are pulling the steel outwards they are trying to split it in half let's give them a chance to do so by turning it into a double piece of steel now when I release my fingers will they actually manage to split it in half ?

They will do it now here is a situation, isn't it in that condition? They just don't want to meet each other, but with a single piece of steel, both magnets are happy to stick to it. I told you permanent magnets were going to be difficult, but believe me, there are worse things to come, suppose I take a non-magnetized one. steel ball and a piece of non-magnetized steel. I'm going to put the steel ball on the magnetic pole as I get closer and touch it with the non-magnetized tip. I can always manage to move the ball away with the non-magnetized piece if I put a piece of steel. of cardboard between the magnet and the ball, for starters, you can see the ball jump away from the magnet.

Now I am going to suggest that an explanation of these phenomena using the concept of pole would be very difficult, so what analogy can I find? Help me with an explanation. I'm going to use an analogue of a magnetic circuit. This is an iron ring and it carries two coils, a large primary coil and a smaller secondary coil connected to an ammeter when I switch a current from a battery to the primary coil there will be a momentary current in the secondary. I'm going to remove the meter and replace it on the secondary with a small lamp.

If now I can change the current in this coil continuously, which I can do by feeding it with alternating current. current then I can turn on the lamp continuously it seems as if the iron ring is acting as a kind of transmitter of something or another between this coil and that one and we call this magnetic flux now let's see what will happen if we take the iron ring completely away now with the secondary coil in the same position as before I turn on the AC as before and now the lamp does not turn on now this magnetic circuit is not exactly the same as the electrical circuit, let's see what happens if I put the iron back but we keep a gap in the circuit Now it seems like we have broken the circuit completely, however when I put the coil in and turn on the air conditioning, we turn on the lamp although it is not as bright as before, so it seems. that our magnetic flux can jump this gap and that air itself is a conductor of

magnetism

, however weak, even if we removed the iron completely there would still be an effect, although it would not be enough to light the lamp. apply these findings to our permanent magnet systems by looking at the permanent magnet quite differently on this board.

I have drawn the lines of force as they would appear from above the magnet there, but I have drawn them on an electrical wire because instead of a source of magnetomotive force I am going to use a battery which is a source of electromotive force I am going to measure the flow of current on these lines on this meter when I turn on the battery now we are measuring the current in All the wires together now remember air is a poor conductor of magnetism so these are high resistance wires. In this context, steel would be a good conductor, which is why it is represented by this piece of copper.

When I put the copper on the wires, the current increases. because I'm shorting some of the high resistance parts and as I slide it onto the pole I cover more and more wires so the current keeps increasing. The law of nature is such that it will always try to reduce reluctance. of the external circuit and this is the rule that we will use now instead of the magnetic poles rule and we can use it to explain all the phenomena of magnets that we saw before. Now you don't have to understand something you can see. To use it as an analogue, suppose, for example, that I wanted to understand the behavior of molecules in a gas that I cannot see using something that I can see now although we do not understand magnetism in the strict sense of the word.

The behavior of magnets is familiar to me, so I am going to use many small magnets to represent the gas molecules. Each of these magnets has been cut from a sheet of rubber that has been impregnated with berium ferrite and then magnetized. in a transparent box on top of a row of electromagnets I am going to put a transparent lid on top of the magnets and then I will turn on alternating current the movement is exactly the same as the movement of gas molecules completely random the lid is lifted due to the impact of individual particles on it and this is what we call the pressure of a gas.

Now suppose I move a plunger from this end and compress the gas to half its original volume while maintaining the same voltage. The lid now rises twice as high. and this is what we call the law of boiling. I can change the temperature of the gas by changing the voltage I supply to the electromagnets. Let's try this. I can increase the temperature like this or lower it like this. Here is another demonstration you can do with this device. If I remove the lid completely, I can show that whether it is evaporation from the surface of a liquid or diffusion of gases, individual particles will have lucky collisions with their companions and will be completely expelled when smoke is introduced into a gas and then examine under a microscope you see bright dancing reflections of light.

This is known as Brownian motion. It is used as evidence of molecular activity, although it is known that the molecules themselves cannot be seen. The smoke particles are being shaken by the molecules. Now I can put smoke. in my gas in the form of paper balls that will not be affected by the magnetic field, put a lid on it and turn it on and there is the Brownian motion represented in the analog. Now I am going to connect the motor in a different way, so that the electromagnets produce their maximum flux at different times and that the entire effect will be that of a traveling wave of magnetic field.

Now let's look at what happens to the particles. We are effectively pumping the gas to one end of the box. Now we can do this. In real life, can we pump gases without a piston? I guess what I'm asking is whether we can operate on individual gas molecules with a moving magnetic field and the answer is that we can, if we can remove an electron from each molecule in this state. The gas is said to be ionized and this entire gas pumping process has the wonderful name of Magnetohydrodynamics. I'm afraid I can't show you mhd as it was reduced to very simple, but I can show that we can pump fluids by at least pumping Mercury.

Instead of gas, this channel has a piece of steel attached to it to improve the magnetic circuit of this motor. When I pour mercury into the channel, it will flow under the piece of steel and it will be pumped along the bottom towards this end and it will flow. Go back to the top now, when I turn on the magnetic field, you can see Mercury forming a small fountain at this end. It's not easy to see Mercury flowing, but you can float a coin in the river and I hope this convinces you that we can pump fluids electromagnetically.

This same magnetic field is much more powerful if it operates on a sheet of aluminum instead of on Mercury. The reason for this is that aluminum is approximately 40 times more conductive than Mercury. Of course, I could make it even more powerful if I could put some steel behind the aluminum to improve the magnetic circuit, but instead I'm going to change the shape of the motor. Can you imagine if the two sides were folded up and everything was rolled up to form a tube? to do let's make a machine that looks like this we must find that the windings are very simple being just a row of coils like this the moving part is a steel rod with a copper sleeve around it we place it at the end of the tube and it fires, well , that wasn't a bad shot.

This electromagnetic gun seems to have taken us back to the beginning with our rifle. It seems we can make an electromagnetic model of almost anything. We certainly managed to reproduce the weapon, but. Can we also reproduce the objective? Can I float a ball in a magnetic field the same way I floated the ball in the stream of water? Well, here's a ball that's made of aluminum and this will act as our jet, a single coil. that will carry alternating current. I turn it on, the ball is certainly trying its best to float, but you see what's happening, it's like trying to balance a pencil on its tip, what we need is some internal force to make it move back.

Every time it tries to decenter there seems to be no equivalent to the surface tension we had with the water jet. Now inside this coil is a second coil and I'm going to feed it with alternating current of a different phase so that I can produce a magnetic field traveling inward that I can detect with our rotating cylinder. Notice that the bottom of the cylinder always moves inward and now when I replace the ball it does. It's one thing to float a big ball like this, but what? kind of apparatus, if we need to float a ball this size, it's a different world, you know, from the world of pingpong balls, Barry, if you just come and pull the ball back slightly, we can see what we really have Now, this is kind of an exaggerated version of the small ball coil the inner coil is placed deeper relative to the outer one first of all, we will send current through the outer coil just like we did before, you would like to turn on the switch Please, Jim, now, Barry. and I have to catch it, otherwise it might be damaged by falling backwards, ready Jim 3 2 one to the right, lower the bar, let's try to balance it, no, Jim again, it's not stable with a single coil, could you connect the other coil , please?

Jim, could you take awaythe ball, Barry, please? Now I am going to check the sequence of faces of the two coils using a rotating cylinder that I have used before. What I want to happen is for the magnetic field to go down the outside of the ball. cylinder and rotate it in that direction, so I want to see the cylinder rotating in that direction. Turn on Jim. Turn off Jim. That is the right direction. Now, bar. If you could have the ball, please, this time, it shouldn't come out. We should be able to see the magnetic flux spraying out the sides like you did with the water from the ping pong ball, it might even spin for us, you will never be able to tell what this ball will do if it spins, then we will have come. full circle in the circle of magnetism ready Jim three 2 1 in oh