i'm playing with a compass needle near a
wire and it was discovered by hans
christian ersted in 1820 that if you
have a direct current passing through a
wire and i've got one here that goes
down under the table
around the table let's have a look it
goes under the table and then it goes
round in a circuit and i've got
well i haven't turned it on yet but it's
going to be 10 volts going through there
leading to a current and ursa discovered
in a public experiment that he did in
front of a big audience he found that
you could deflect the direction of the
compass needle
i've got my compass needle the red
points towards the south the silver side
goes towards the north now i'm going to
turn this around the wire and you can
see as i do that it still points in the
same direction
as soon as i turn on an electric current
earth did found that the direction of
the needle will move
so there it is that's pointing north
north is that way and i'm going to turn
it a bit further so at this stage it's
pointing it seems east but that's
actually north
and now i'm going to go to the voltage
supply and put 10 amps through this wire
and you can see as you turn that on the
compass needle no longer points east it
points more towards this direction and
i'll turn it off
and i'll turn it on
and you can see that it ends up being
deflected now you can think of this as
being a magnetic field this way
and a magnetic field pointing that way
and the sum of these two magnetic fields
i can represent by saying that's the
earth's magnetic field and this is a
much bigger magnetic field at right
angles to it because i've turned this
around by 90 degrees and that points in
this direction so the compass needle
tells me the direction of this resultant
magnetic field the sum of these two and
it gives a direction
and now if i turn this round
by another 90 degrees
so i probably haven't got it lined up
exactly right but now i've got a
magnetic field given by the green
pointing that way and a magnetic field
roughly pointing in the opposite
direction
and this is
smaller than the magnetic field due to
the current so i get a little magnetic
field pointing that way so it's pointing
in the opposite direction now if i move
it around a bit further
you can see
the direction change
and then when i go all the way around
back to the
starting point the magnetic field seems
to have turned around curled around and
come back into the same direction
so what ursula discovered was the
magnetic field near the wire
circulates around the wire as though
it's going in a circular path it's
bigger close in and it's weaker further
out there is a relationship by somebody
else called ampere which relates what
happens as you measure the magnetic
field round here times the distance and
that's related to the current in the
wire which way is the current flying we
have a rule if the current is flowing
upwards
you point your thumb in that direction
and then your fingers curl around the
wire and that tells you the direction of
the magnetic field so there's something
called the right hand rule to tell you
the direction of this thing so you're
seeing the first example
through this experiment of how a compass
needle is moved by a current now you
from that you can build a way of sending
messages down wires you start off with a
current which will then deflect a needle
one way and then it goes back to where
it was
deflected and go and this is the
beginning of telegraphy sending
electrical messages down wires dots and
dashes if you like and then you can send
information somewhere else if you ever
see the the movie the general with
buster keaton from way back it's meant
to be in the civil war period which is i
don't know 19th century and they had
telegraph wires alongside the railroads
and the way you could set information
was through these telegraph wires down
to let people know what the army was up
to so buster keaton spent his time
pulling down all the telegraph wires to
make sure that the messages couldn't get
through somebody doing morse code dot
and dash well in this case it's going to
be on or off and i could do dot
and then i have to wait and dot
and then there's a weight and then
another dot
and then i could have a dash which would
be
longer so we have it on there for a
longer period
and then off
i'm making this up as i go along this is
the beginning of sending messages
it's a turning point in science it's
more of a turning point in science
because this is an electric current
passing down a wire producing a magnetic
effect so
with this discovery ersted had
discovered a connection between
electricity and magnetism they weren't
separate subjects after this they were
part of the same subject which we now
call electromagnetism
you know what today i'm going to curl to
the left and not the right it i don't
see why it always curls the same way
that's the way nature is
what can i tell you