oh well you can't win them all can you
now what am I a professor of electrical
engineering doing potting at ping pong
balls it's not just that I think science
ought to be fun I do have a serious
purpose in mind and it isn't the
shooting part that interests me it's how
these balls stay suspended on the water
jets that really fascinates me
you could ask of course what an
electrical engineer is doing doubling in
Hydraulics anyway and the answer is that
I can see what is actually going on you
see my own subjects are electricity and
magnetism and both of these are
invisible I'm trying to make a model of
what might actually go on in a magnetic
circuit which behaves in exactly the
same way as this water circuit in other
words I'm trying to make the invisible
visible now the thing about analoges is
that you need to know just how far they
can be useful and just when you have to
shoot them down I want you to see this
as a circuit see when the water has hit
the ball it pulls back into the trough
and then is returned through pipes back
to the pump now of course we can make
the whole thing neater by enclosing it
all in pipes like
this in this circuit the driving member
is here the pump this is a valve or
switch when I open the
valve you will see a plunger rise in
this tube here this is recording the
rate of flow of the
water here is a manometer recording the
pressure
difference between two points in the
circuit the manometer is connected in
here and in here so that it measures the
pressure in this piece of
pipe there and this water circuit has
been designed to be the analog of an
electric 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
ameter this voltmeter can be connected
either to that
point or to this
point and by subtracting the two
readings we can measure the electrical
pressure drop along this part of the
coil so I'm using something that I know
about to help me explain something I
know less
about now the trouble with analogs is
that none of them are ever completely
true so let's see how far we can take
this water electric analog before it
breaks down first of all if I replaced
this rigid pipe with a flexible hose
then when I turn the pressure on it
would blow out like a garden hose pipe
does and try and make itself into a
circle now will the same thing happen
with the electric Circle
suppose I were to replace this coil with
a more flexible one what do you think
would
happen I'm going to use a loop of
aluminium foil ordinary kitchen foil
will do but the thinner the better when
I switch on the
current you see it does bow out like the
water pipe I mustn't leave the current
on for too long each time otherwise I
shall burn out 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
alongside and this time when I switch
on something very different occurs and
of course there's no equivalent of this
in the water circuit unless I can get
hold of a water divver and his Hazel
twig but then you know you always get
into difficulties when you try to
explain the behavior of permanent
magnets let me show you what I
mean I'm going to use two bar magnets
and first of all I place them with like
poles opposite so of course they repel
each
other if I put a bit of Steel now on the
pole of that magnet I don't change the
polarity of it by just doing that if
that was a North Pole before then the
outside of the steel will still be a
North Pole and so it should still repel
the other magnet shouldn't
it surprise
surprise now in that condition both
magnets are pulling on the steel
outwards they're trying to split it into
in half let's give them the opportunity
to do that by making it a double piece
of
Steel now when I release my fingers will
they in fact succeed in pulling it in
half they do now here's a situation
isn't it in that condition they just
don't want to know each other but with
only a single piece of Steel both
magnets are quite happy to attach
themselves to
it I told you permanent magnets were
going to be difficult but believe me
there's worse to follow
suppose I take an unmagnetized steel
ball and an unmagnetized bit of
Steel I'm going to put the steel ball on
the magnet pole as I approach it and
touch it with the unmagnetized bit I can
always succeed in bringing the ball away
with the unmagnetized piece if I put a
bit of cardboard in between magnet and
ball to begin with you can actually see
the ball jump away from the
magnet
now I'm going to suggest that an
explanation of these phenomena using the
pole concept would be very difficult
indeed so what analog can I find to help
me with an
explanation I'm going to use an analog
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 into the primary coil there will
be a momentary current in the
secondary I'm going to remove the
meter and replace it across 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 alternate ating current then I
can light the lamp
continuously it seems as if the iron
ring is acting as a sort of transmitter
of something or other between this coil
and that one and we call this something
a magnetic flux now let's see what will
happen if we take the iron ring away
entirely now with the secondary coil in
the same position as before I switch on
the AC as before
and now the lamp does not
light now this magnetic circuit is not
quite the same as the electric circuit
let's see what happens if I put the iron
back but keep a gap in the circuit now
it looks as if we've broken the circuit
completely nevertheless when I put on
the coil and switch on the
AC we light the lamp even though it's
not as bright as it was
before so it seems that our magnetic
flux can jump this Gap and that air
itself is a conductor of magnetism even
though a weak
one even if we take the iron away
entirely there would still be an effect
even though it wouldn't be enough to
light the
lamp now let's apply these findings to
our permanent magnet systems looking at
the permanent magnet in a quite
different
way on this board I've drawn out the
lines of force as they would appear from
above magnet
there but I've drawn them out in
electric wire because instead of a
source of magnetomotive force I'm going
to use a battery which is a source of
electromotive
force I'm going to measure the current
flow in these lines on this
meter when I switch on the battery we're
now measuring the current in all the
wires together now remember that air is
a bad conductor of magnetism so these
are high resistance ones
wires in this context steel would be a
good conductor so it's represented by
this piece of copper when I put the
copper down on the wires the current
increases because I'm short circuiting
some of the high resistance parts and as
I slide it round towards the pole I'm
covering more and more wires so the
current continues to
increase the law of nature is such that
it will always try to reduce the
reluctance of the external circuit and
this is the rule we shall now use in
place of the rule of magnetic poles and
we can use it to explain all the
phenomena of magnets which we saw
earlier now you don't have to understand
something which you can see in order to
use it as an
analog suppose for example that I wanted
to understand the behavior of molecules
in a gas which I can't see using
something which I can see now although
we don't understand in the strict sense
of the word magnetism the behavior of
magnets is familiar to me so I'm going
to use use lots of small magnets to
represent the gas
molecules each of these magnets has been
cut from a sheet of rubber which has
been impregnated with berium ferite and
then
magnetized they're in a transparent box
on top of a row of
electromagnets I'm going to put a
transparent lid on top of the
magnets and then switch on alternating
current
the movement is exactly the same as the
movement of gas molecules entirely
random the lid lifts because of the
impact of individual particles on it and
this is what we call the pressure of a
[Applause]
gas now suppose that I move in a plunger
from this
end and compress the gas into half its
original
volume
keeping the voltage the
same the lid now Rises twice as
high and this is what we call boils
law I can change the temperature of the
gas by changing the voltage that I
supply to the
electromagnets let's try
this I can raise the temperature like
this or lower it like
this here's another demonstration you
can do with this apparatus if I take the
lid off
entirely then I can show either
evaporation from the surface of a liquid
or the diffusion of
gases individual particles will have
fortunate collisions with their fellow
particles and be ejected
entirely when when smoke is introduced
into a gas and then examined under a
microscope bright flexs of light are
seen dancing about this is known as
Brownian motion it's used as evidence of
molecular activity even though it's
known that the molecules themselves are
not being seen the smoke particles are
being buffeted about by the molecules
now I can put smoke into my gas in the
form of paper balls which will not be
affected by the magnetic
field put a lid on and switch
on and there is the Brownian motion
represented in the
analog now I'm going to connect up the
motor in a different way so that the
electromagnets produce their maximum
flux at different times and so that the
whole effect shall be that of a
traveling wave of magnetic field
now watch what happens to the
particles we are effectively pumping the
gas to one end of the box now can we do
this in real life can we pump gases
without a piston I suppose what I'm
asking is can we operate on individual
gas molecules with a moving magnetic
field
and the answer is that we can if we can
strip an electron off each molecule in
this state the gas is said to be
ionized and this whole process of gas
pumping has the wonderful name Magneto
hydrodynamics I'm afraid I can't show
you mhd as it shortened to very simply
but I can demonstrate that we can pump
fluids at least by pumping Mercury
rather than a
gas this trough has a piece of Steel
fixed into it to improve the the
magnetic circuit of this motor when I
pour mercury into the trough it's going
to flow underneath the piece of Steel
and be pumped along the bottom up this
end and flow back over the
top now when I switch on the magnetic
field you can see the Mercury making a
small fountain at this
end it isn't easy to see Mercury flowing
but you can float a coin on the
River and I hope that this convinces you
that we can pump fluids
electromagnetically this same magnetic
field is much more potent if it operates
on an aluminium sheet instead of on the
Mercury the reason for this is that
aluminium is about 40 times more
conducting than
Mercury of course I could make it more
potent still if I could put some steel
behind the aluminium to improve the
magnetic circuit but instead of this I'm
going to change the shape of the motor
can you imagine that the two sides were
bent upwards and the whole thing rolled
up into a tube having imagined that to
be done let's make a machine that looks
like
that we should find that the windings
are very simple being only a row of
coils like this the moving part is a
steel rod with a copper sleeve around it
we put it in the end of the
tube and
fire well that wasn't a bad shot was it
this electromagnetic gun seems to have
taken us right back to the start with
our rifle it looks as if we can make an
electromagnetic model of nearly anything
we' certainly managed to reproduce the
gun but can we also reproduce the target
can I float a ball on a magnetic field
in the same way that I floated the ball
on the water
jet well here is is a ball it's made of
aluminium and this is going to act as
our jet a single coil which is going to
carry alternating
current I switch 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
Point what we need is some inward Force
which is going to make it drift back
every time it tries to get off center
there seems to be no equivalent of the
surface tension we had with the water
jet now inside this coil there is a
second coil and I'm going to feed this
with alternating current of a different
phase so that I can produce an inward
traveling magnetic field which I can
detect with our rotatable
cylinder notice that the bottom of the
cylinder is always moving
inwards and now when I replace the
ball it
does now it's one thing to float a ball
as big as this but what sort of
apparatus should 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 take
the ball back slightly we can see what
we've actually got now this is a sort of
exaggerated version of the small ball
coil the inner coil is set down deeper
in relation to the outer first of all
we're going to send current through the
outer coil only as we did before you
like to come on the switch please
Jim now Barry you and I have got to
catch it otherwise it might damage
Itself by Falling Back ready Jim 3 2 one
on right lower it bar
let's try and balance
it no off Jim again it isn't stable with
a single coil could you connect the
other coil please Jim and would you take
the ball away Barry
please now I'm going to check the face
sequence of the two coils using a
rotatable cylinder which I've used
before
what I want to happen is the magnetic
field to go down the outside of the
cylinder and so rotate it in that
direction so I want to see the cylinder
spinning that way switch on
Jim Swit off Jim that is the correct
direction now bar if you could have the
ball
please now this time it shouldn't drift
off this time you should fairly be able
to see the magnetic flux spraying on off
the sides like you did the water from
the ping pong ball it might even spin
for us you can never tell just what this
Ball's going to do if it spins then we
shall indeed have come full circle in
the circle of magnetism ready Jim three
2 1
[Music]
on
oh