the whole effect is a subject today and
you might think the Hall effect is
something to do with the echo in this
hall that's what at first sight it seems
to be but it's a thing named after Edwin
Hall who invented the idea in 1879 but
you could actually get some funny
effector and magnetic field near a wire
so let me show you about that
before I talk about the Hall effect let
me explain something else which is
called the Lorentz force and in this
apparatus I have electric charges coming
out of here and they ionize the gas and
make it glow green so you've probably
seen something like this before
now this is a magnet a really strong
magnet so this is put in a little I
don't know thirty six millimeter camera
so that it doesn't clang again to other
things and if I put that close to this
you can see that the beam of electrons
coming out of there giving that green
glow is deflected it goes up if it's
near this pole and it goes down if I
turn it around for the other Pole so the
effect of a magnetic field if the
electrons are running this way is to
bend it upwards now I could do the same
experiment this one and now I put a
sheet of metal this is aluminium and I'm
going to put this sheet of metal in the
way of the magnet have you got the beam
still there Brady because I'm going to
put this close to the metal and I'm not
cheating you take the metal away it's
still there put the metal there it's
still bending the magnetic field goes
through the metal any metal when it's
behaving normally will allow magnetic
fields to penetrate it
the question posed by Edwin Hall was
what happens if you have an electrical
current down a wire as in these wires
but I've blown it up so you can see it
quite clearly and I put a magnetic field
near it now then you've seen already
that if there is a charge running along
in this direction a magnetic field will
make it Bend so the magnetic that charge
is going along here and it's going to go
and Bend and suddenly hits a wall so if
I now think about a wire such as this
without a magnetic field and let's
pretend this is a charge carrier so you
can really see what's going on I mean
this is not really to scale here's the
bit of the wire and the charge goes
through here flowing down the wire and
the current is just the number of
charges moving down the wire per unit
time so if I put a magnetic field down
here and the charges are coming along as
you've seen
this experiment it's going to bend up
and come to rest on the edge of the wire
and if I do this straight off and then
send another one down if ii will do this
when i got negative charges there so
i'll cancel them out with positive
charges over here and down will come
another one and it will suddenly say oh
i want to go here and there will be a
positive charge induced on the other
side so now when another charge comes
along it wants to be attracted this way
because it wants to go near the positive
charge but on the other hand there is a
magnetic field which wants to sweep it
that way and the balance of these two
means that it doesn't choose to go that
way or that way it just carries on down
the middle
not being attracted to either side and
as a result after these charges have
built up on one side and the other it
gets to the steady-state so more and
more charges can come through just as if
this wasn't there so there's no reason
to expect that there's any effect at all
the current carries on flowing it's just
like water going down a tube you don't
know that there's anything happening on
either side the upshot of all this is
the magnetic field does not affect the
current going through the wire but you
now get between the negative charge and
a positive charge a difference in
electric potential which you can measure
using a suitable meter so the idea is
this you send a certain current through
here and you measure this voltage from
one side to the other this is not the
same as Ohm's law where it's the voltage
down the wire it's the voltage across
the wire that you're measuring it's
quite different and this is the idea
that Hall came up with so you can
measure this voltage divided by this
current which has a unit of resistance
because it's a voltage times the current
but it's nothing to do with Ohm's law
this is the Hall effect of voltage this
way with a current flowing that way you
probably want to know Brady what the use
of this is well I can get some very
clever modern electronics if I can turn
this thing on and you know how I am
Brady I'm a theoretician I touch pieces
of apparatus and they break down so this
might take a little while to to work out
right at the end there
there's a tiny little square about a
millimeter square or bit bigger and
wires go into and out of it and that is
the whole probe this small little square
is the analog of this big one and there
are currents going into it and there is
a voltage across it so if I put this in
a magnetic field and I put that in there
and you suddenly get oh it's gone off
the screen you can measure this in Tesla
that's eighty-eight point that's a
hundred hundreds of military in there
and you get in the middle it goes off
the scale so you can actually wander
around the room with this and measure
the magnetic field in any particular
direction so if I have the square like
that it's a magnetic field coming in
perpendicular that you're measuring if I
twist it round I get a different
magnetic field so you might want to
invent things using this and one of the
things which is used using this effect
is a burglar alarm you put a tiny little
magnet in your door apparently and a
hole probe in the jamb and set it all up
with electronics and as soon as the
burglar comes in the door this moves
away from the magnet and there's a huge
change in the magnetic field and you get
a nice current and nobody sees that
you're doing it