[Music]

at the end of my last talk I described

par paradise's idea of a magnetic field

how the presence of the magnet made a

difference in the space around which

gave us magnetic effects and how we

could map out the direction of the force

the magnetic force by the lines along

which ACC Composites Itself by means of

these filings I've just tapped the

filings again and you can see this

Faraday field around the magnet here in

the middle now I don't know whether you

get the impression that these lines are

not starting from each end and running

round they are running round and then

diving into the magnet and running round

again and that's the right impression

you ought to get this field runs round

like that and round like that into the

magnet again this is rather like they

always look be are like traffic lines

you could imagine this is lines of

traffic out in the open and this is a

kind of underpass like you have at hi

Park corner where where the traffic

Dives in and goes into the iron and then

comes out again at the other end well I

want to show you that that there really

is a field inside the iron as well as in

the space outside it I'm going to do

that with a saw blade here that saw

blade is at the moment not magnetic if I

take it and dip it into these little

iron filings you see it's quite

nothing in the

middle it's quite inert there you see

it's quite inert but this is is really

there but the lines of force are inside

cuz if I take that saw blade and snap it

in two you see a magnet a magnet the

lines of force are coming out into the

open now and showing us they were really

there inside the whole

time well now so far we've been talking

about magnets and we've mentioned the

load Stone and I've used other magnets

here but we must remember that when this

all started there really was no way of

getting at all a strong magnet load

stones are quite feeble magnets one

could make a piece of Steel into a

magnet by stroking it with a load Stone

like I did that saw blade but it was a

very feble magnet when that was done and

it was a very great discovery in the

history of magnetism when for the first

time it was found out by Earth dead

early in the 19th century that you could

make a magnetic field with an electric

current that the flowing of a current

along a wire made a magnetic field in

its neighborhood a new way of making a

magnetic field Earth Dead's experiment

we've got rigged up here he showed this

was his great discovery that if you have

a wire along which a current can be sent

and a compass needle just under the wire

that when one switches on and sends a

current along this wire it tends to

drive the North Pole the red pole in One

Direction and the other South Pole in

the opposite direction in fact to twist

the magnet around so it tries to sort of

set itself at right angles to the way

now Mr coat will switch the C on and you

see the magnet trying to set itself we

stady it

again s c on again there you see how it

how it switches around the current

obviously makes a magnetic field there

pushing this pole this way so in that

way we create a magnetic field by means

of a current now perhaps you can see if

a current running along here pushes that

North Pole that way it'll try to push it

this way and this way and this way by

symmetry around that wire so it was soon

realized that the way to get a really

strong magnetic field was to send a

current round and round a

wire suppose you take a piece of iron

like this and wrap a wire around it then

you see the current running along here

will tend to make magnetic field that

way just it did in the earth experiment

the current running along here will do

the same thing the current running along

here will do the same thing and so on so

all the current around all these turns

of wire tends to make a magnetic field

the same way

here you see this piece of iron is quite

unmagnetized up quite a lot of the nails

turn the current off and down all the

nails go except for a little magnetism

left in the arm you'll see why that's so

uh presently so 's discovery made it

possible to make really strong magnets

for the first time by sending electric

currents round coils of wire having iron

in the middle uh we have for instance uh

a very tiny magnet of that kind uh it

this magnet is hanging up here on this

Gantry it's the wire runs round and

round in a coil outside is a kind of

shell of iron and there's a piece of

iron in the middle now we've got a

little keeper Mr coats has here which

you see doesn't attracted at all but if

we send a current around that wire we'll

make the central piece of iron into a

magnet and it'll hold on its little

keeper this dry battery is all I need to

send enough current along and you'll see

it then becomes a very strong magnet

indeed now first of all I'm not sending

the current through you'll see there's

no stion at all but now if I connect

this on and send the current through it

has really made it into quite a strong

little magnet watch my assistant now and

I when he says off I'll turn off the

current

off we've said a piece of ir we say

piece of iron is attracted by a magnet

Thinking of it in terms of a field what

we really ought to say is a piece of

iron tries to go from a place where the

field is weak to the place where the

field is strong now here again is one of

these solenoids these coils to which we

can send a current here is a piece of

iron at the bottom here it's resting on

the table but if my assistant switches

on the current you'll see that iron try

to get into the strong field

on off

on off so that's the right way to think

magnetic

attraction finally as you saw when I

stroked a piece of iron a piece of iron

can be a magnet or not this piece of

iron is quite inert doesn't pick up any

nails at all if I just stroke it with a

magnet once like that is enough to turn

it into quite a strong

magnet now here I've got what we call

our Magic Pot and we can make a piece of

iron into a magnet or not a magnet in

that pot as we like to show how easy it

is for iron to be in either State should

we have a magnet first

you see it's quite a strong magnet now

please we'll have not a

magnet nothing you see a magnet

again and not a

magnet inert now I'm going to plunge

rather more deeply into the inner

structure of the iron and think why we

see all these effects I started the

lectures by saying a magnet is a magnet

because it's made of little magnets and

that is literally true in Iron which is

of course built of atoms every atom is a

small magnet and we can get a good deal

further if we think of this structure

these little magnets influence each

other and in fact in the iron they all

group themselves into families what we

call domains uh areas in the iron inside

Each of which the magnets are head to

tail north to south all pointing the

same way and we can understand this and

how it affects the behavior of the Iron

by a model I'm going to show you now

quite a famous model the Ying

model the Ying model I've got one here

is a lot of little Compass needles all

ped so they can turn around freely

they're all magnetized North and South

they got little arrows white arrows at

their North ends now if you leave such a

model like this alone give it a stir up

and it settle down by itself you will

see how these little magnets break up

into domains just like we're supposing

our AR to do for instance all the little

magnets there are pointing this way all

those there pointing that way here is a

little family pointing that way and

there's a little family over there

pointing that way and that tells us once

what the difference is between a piece

of iron when it's a magnet and a piece

of iron when it's not a magnet you saw

we could turn from one to the other

quite

easily if we put the iron in a magnetic

field we say it magnetizes it what does

it do field from the magnet makes all

these little magnets turn round and

point the same way and then the iron of

course will behave as if it were a

magnet I've got such a magnet here and

I'll slowly approach it to this you'll

see these little pillows begin to get

Lively and is it wor think bitter of it

they must do something about it and as I

get it closer and closer it will make

them all point in the same direction you

see them now all now I've magnetized

that AR as we say we've turned it into a

magnet by putting it a magnetic field if

I alter that field which I can do by

turning this magnet round you'll see

they get unhappy

again they're trying to follow the new

direction of the

field I turned it quite round they'll

all reverse Direction and try and point

the opposite way to what they were doing

before they're there near they're doing

it you see so that is an explanation of

the way in which magnetic field turns a

piece of iron into a magnet now you may

have noticed when I turned that round

they were sort of sticky they didn't all

do what they they were supposed to do at

once and that was because uh there are

certain friction as it were in the

magnets turning round we can help that

friction by giving them a bit of a knock

by jigging them so to speak and help

them to make up their minds and I have

here uh quite a well-known experiment

always known as the poker experiment

just to show that if we give these

little magnets a chance to orientate

themselves by giving them a little

movement how quite a weak field is able

to turn them around and make them all

point the same

way here is our compass

needle here is an ordinary poker which

we've carefully demagnetized now if I

approach the end of that poker to either

pole of this Compass it will attract

because the compass is a little bit

magnetic you see and tends to attract it

same way that end there it attracts

either end attracts the north towards it

you see because the magnet of the

compass is pulled towards the iron of

the poker now if I take this poker place

it in the direction of the Earth's field

which is trying to pull its little

magnets around but is very very weak

indeed if I give it a hearty

bang now I hope we'll see we've turned

this into a magnet and where before it

attracted the pole

now it repels it do you see pushes it

away so we've made this magnet simply by

giving it a hearty knock with the

hammer here is a very extraordinary body

um this body here is what we call

permalloy and this uh permalloy is so

soft that we don't even have to give it

a hit it magnets turn each other around

just in the earth's

field I'll hold it first of all at right

angles to the Earth's field and we'll

take our magnet right away and then you

will see it's quite

non-magnetic uh it these little magnets

here really don't take any notes of it

as I move it

about but now if I hold it so it's

pointing in the direction of the Earth's

field which is about this here here now

do you see it all quite

excited I've simply made it a magnet by

turning it around the Earth's field and

made it a magnet Again by simply putting

it at right angles to the Earth field

there do see it's as soft as

that so in a body like this little M can

turn around very easily and we call it

very soft magnetically at the other hand

there's the bodies of which we make our

permanent magnets now there these little

Compass needles these Atomic magnets are

very hard to turn around there's a great

deal of friction but once they're turned

it's very hard to turn them back again

so if you put on a big magnetic field

and then take it away it remains a

strong magnet we've got one of those

really strong magnets here we'll hang it

up by its keeper on a Gantry and you

will see it will stand quite a

weit and now just to show that there's

no cheating about it uh Mr coats will

wrench all its keeper I want you to make

sure it really isn't fixed on there it

is you see there is the keeper of this

strong

magnet well I'll end with what I began

with uh a magnet is a magnet because

it's made of little magnets and I think

you'll realize if we think of it in that

way then we have a much better

understanding of the way magnets work

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