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so far I've been talking mainly about

the magnetic properties of an which is

the most striking and best known of all

the magnetic bodies but there are other

bodies which are not quite so magnetic

if we can put it that way as an but

quite distinctly so uh an's two

companions in the periodic table nickel

and Cobalt are quite strong magnetic

bodies here is a piece of iron and of

course my magnet sticks to it like

anything and picks it up here's a piece

of pure

nickel you see this is quite magnetic

too and here are some little chunks of

cobalt which are picked up by the

magnet most mles however are not

magnetic in this way at all piece of

aluminium absolutely inert and here and

this is interesting here's a piece of

stainless

steel although it's nearly all Iron it's

an alloy of iron to make it stainless

you see it is not magnetic either I

might perhaps say that this is rather

old-fashioned stainless steel they make

it more cheaply nowadays it's got more

iron in it and if you try it on one of

your stainless steel knives you'll

probably find the magnet will pick it

up well those are all what are called

ferromagnetic bodies iron Cobalt and

nickel strongly mag

magnetic now on the other hand there's a

whole class of bodies which are magnetic

but very very weakly so they only show

that they want to get from a weak field

into a strong field but the force is

extremely

small it's not that just some are a bit

more so and some are a bit less so

there's a whole class of difference

between the perom magnetics like iron

and nickel and the magnetics as they're

called which have this feeble

effect for instance salts of iron a

solution of iron salt is very very

feebly magnetic in the sense that if you

have one of these Solutions of iron it

tries to move from a place where the

field is weak into a place where the

field is strong just like a magnetic

material like ad does here for instance

I've got in this tube this u tube a

solution of fic chloride and one limb of

the tube is between the poles of this

very powerful electromagnet and I think

you will see if I switch the magnet on

by mean of this switch here the liquid

will be sucked up between the poles

showing it's trying to get into this

strongly magnetic

field there you go up down again on and

off so that is a feebly magnetic body

oxygen shows this property rather

interestingly it's hard to see with

oxygen as a gas because of course it's

so tenuous but when we have oxygen in

liquid form then you can see it is quite

a magnetic body I've got a pot of liquid

oxygen here here we got a very powerful

permanent magnet one of those made in

the war for the magnetron a radar device

which needed a very strong field between

the poles here if I take this pot of

oxygen and pour it between the poles and

you'll see it sticks in the strong

magnetic field watch

now you see it boiling away between the

poles it boils away of course cuz they

pose for it are very very hot

indeed why this difference it's not

because in these two classes of body and

by the way we call these perom magnetic

ones that are like Aron nickel and

Cobalt and paramagnetic the very weak

ones is not because we haven't got

little Atomic magnets in the

paramagnetic ones the atoms are little

magnets just as they are in iron the

difference in them comes about in this

way in a body like iron these small

magnets are like the magnets of our uing

model they influence each other so

strongly that they all tend to line up

in rows in domains pointing the same way

the North Pole of one pointing to the

South Pole of the next the same magnets

are there in the paramagnetics but their

influence on each other is

feebler and there isn't this tendency

for them all to arrange themselves in

rows now when we've got domains and

groups of atoms all in rows it's much

easier to magnetize that body than it is

when the atoms are or higgledy piggledy

where they don't influence each other so

as to stick in rows perhaps I can

explain what I mean by an analogy you

know how a clever dog can say drive a

flock of a hundred sheep down the road

why is it so easy for the dog because

sheep behave in a sheepish way all of

them point the same way if one sheep's

doing something all the rest are doing

the same so it's very easy for the dog

to get the whole lot to point the way he

wants them to

go compare that with a job a dog would

have if it tried to drive a flock of a

100 cats along the road now cats are

orientated by dogs just as much as sheep

are they turn tend to turn away from the

dog but they're independent they don't

work in domains like sheep do each cat

has its own idea of where it wants to

point and so all though in general the

cats will run away from the dog they

won't go along as a neat little flock in

other words sheep are fom magnetic and

cats are

paramagnetic now here's another point

about these magnetic bodies iron is

strongly magnetic as we know but if you

heat a piece of iron red hot it ceases

to be

magnetic

why well we can illustrate that again

with this Ying model I I've got the uing

model

here and near it I've got a strong

magnet and this magnet is magnetizing

the model it's making all little arrows

point from here towards

here the reason why above a certain

temperature which we call the cury point

the iron ceases to be magnetic is that

the heat motions which are always

disturbing things which are a force of

disorder the heat motions tend to

wriggle these little atoms more and more

violently until finally the domains are

broken up now I'm going to illustrate

that I've got a screwdriver here which

is a just a little bit magnetic as all

the tools in the workshop always are and

I'll simulate heat motion by wriggling

the screwdriver over the model so as to

stir these all up and you will see we

break up the domains

that's what heat does now our iron is

red hot it ceased to be magnetic I let

it cool down by taking my screwdriver

away and they'll all settle down into

the domain again that is to say it has

now become magnetic

again I can illustrate this by piece of

Red Hot Iron that's not so convenient to

have on the elra bench so I've got here

some metals which have a much lower cury

point the cury point you remember is the

point at which the heat

Motion makes them into non-magnetic

materials now this one is called monil

metal and it's around the rim of a wheel

the wheel is here between the poles of a

very strong magnet which is pulling the

rim there and the rim there equally if I

warm up the rim on this side raise it

above its cury point it will cease to be

magnetic the magnet it will no longer

attract the rim here it'll go on

attracting that side and you will see

the

result the rim as you see will go round

and

round simply

because I'm destroying the magnetic

property on this side of the rim finally

here's a mdle with so low a cury point

that it loses its magnetic property even

the temperature of boiling water you see

this this is quite

a quite a btic material you see when I

swing it I just dip it in the boiling

water let it get really hot and now do

you see it's quite

inert put it in cold water

again it's got back its magnetic

property now I'm going to talk about a

subject that's always interested me very

much I find fascinating that is the

question of the Earth's

magnetism we've said that the Earth

behaves as if it were a large magnet

that's why Compass Points North uh what

we find of course is that if we plan the

direction of the magnetic force over the

surface of the Earth where we can

measure it it has a the form that we'd

expect if there were a large magnet

right at the middle of the Earth

here I've got a picture of the lines of

force that there would be around such a

magnet here if we suppose there's a

magnet the center these are the sort of

lines of force which it would have and I

by this red circle I've shown the

surface of the Earth and you will see

that for instance this sort of latitude

the real direction of the lines of force

dipped rather steeply into the ground uh

where we are here in England at the

North Pole itself the magnet would Point

straight down into the ground at the

equator on the other hand the lines of

force are horizontal and then as we go

into the southern hemisphere here of

course we come round again to the South

Pole and there the North Pole of our

Compass would Point straight up in the

air here is a mockup let's see it

happening actually with a model of the

Earth a globe here and at the center of

this we have got a magnet and I've got a

compass needle here what's called a dip

Circle because the angle with which it

points down into the ground is called

the dip a dip Circle which can move

around in a vertical plane when I place

it down towards the South Pole it's

dipping steeply into the ground the

South Pole of the magnet now as I bring

it up do you see it's coming

over when I get about to the

equator it points pretty well

horizontally and as I go up into the

northern part of the world now the North

Pole is pointing down into the Earth and

when I get right up here of course at

the north Magnetic North Pole the

compass needle points straight down into

the ground it turns over as we move it

around the world so you see if we were

in

a closed room but were allowed uh to

have a dip needle we could tell what

latitude we are in just by seeing the

angle with which the dip needle pointed

down into the ground a study of this dip

in past ages has told us a great deal

about the history of the world history

of the

earth about 40 years ago there was a

famous uh German scientist vager who put

forward a very novel and startling

theory he said the continents have not

always been where they are now they have

drifted about all over the place on the

surface of the world rather like large

ice flows floating on a sea the

continents being solid and floating on

the kind of gooey stuff heavier stuff

underneath so they could very very

slowly move he was led to this by the

queer things we find about the parent

climate in past ages for instance coal

it's obvious the kind of plants that

made our coal could only have grown in

the tropics in a uniformly warm

climate yet you find cold in England and

even in

spitsberg Magnolias fossil Magnolias are

found in

Greenland and at the opposite end the

rocks in India show scratches which

could only been made by great ice sheets

coming over India from the south

although India is now north of the

equator so he said it's not because the

climate in spitsbergen was once tropical

but because spitsbergen was once in the

tropics it's moved from down there up to

where it is now there was a great deal

of disbelief of such a novel Theory but

now magnetism has come to rescue and

Vaga has been proved abundantly right we

saw it with our little Ying model that

if the atomic magnets settled down in a

magnetic field they tend to point in the

direction of that field now that happens

in the Rocks When lava cools there are

Atomic magnets in the lava and the lava

very feebly takes up the magnetism of

the earth and shows the direction of the

field at the time it cooled so by

getting pieces of lava from past

geological ages one can measure the dip

if the lava hasn't shifted its position

and tell what latitude that country was

in when the lava was formed and of

course also you can tell which way

around that country pointed because you

know where the North Pole is so you see

now we can trace back the history of the

continents it's an absolutely

fascinating story it's clear that veano

was right they have moved a great deal

it's also clear

that in the course of the

ages at first there was only one

continent all the land that we know now

was joined together in one piece and

cracks have formed and it has split

apart and made the separate continents

as we know them now magnetism has shown

us how that has happened this is the

world more or less as it is now you'll

recognize North America South

America

Africa Europe we've left out Asia and so

on because the Distortion would be too

great and here of course Greenland now

we' got to take us the shape of the

continents not the dry land that you see

in red but the edge of the continental

shelf you probably know that the land

Runs Out under a very shallow sea for

some way and then suddenly drops very

steeply into the deep sea if we take the

outlines of the Continental shells you

will see the fit is almost fantastic

that's the way for instance that South

America fitted onto Africa

and North America America fitted on here

like

this there's a little bit loss there

that's accounted for by Odd West Indies

and so on Europe fitted in

here and that left a place here into

which Greenland very neatly fits it's

just like a jigsaw puzzle so the simple

explanation has now come through

magnetism which explains the Curious way

the climate has altered in the past a

very very fascinating and lovely Theory

well that concludes my series on

magnetism and I hope these

demonstrations will show you that

there's a good deal more to magnetism

than just picking up pins

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