This is two amp fuse wire
it's copper, tinned copper wire which used to be used in old fashioned fuses
and George has this long bit of fuse wire, which is hung between two electrodes.
You probably can't see so we put some paper on this wire
and you see the line across there and then it comes along here
Well this is a wire which is jittering a bit because ... I don't know why ..
you do just leave me ...
Allright it's hanging there in a shape which doesn't seem to be particularly special.
Now I am going to turn the current on
This is volts going up and this is the current in amperes.
So it goes all the way up and this goes up to 1.8 amps roughly, 1.75
So this is below the point where the fuse wire will melt.
So now, I am going to do the experiment. I am going to turn the current on
and here it goes up to 1.8 amps
and now I am going to turn it off.
on
off
It's reversible: it goes up, it goes down.
on
off
on
off
When the current's on, it goes through the fuse wire
and because of the resistance of the fuse wire
it heats up - that's what a fuse wire is for
it's meant to heat up and melt when you get up to about 200°C
this gets up somewhere near there.
so we are not melting this
We are just heating it up quite a bit
and as we heat it up it expands
and you normally can't see the expansion of a metal when you heat it up
but here you can
So this sagging in the middle is due to the fact that the wire has got
a little bit longer. Less than 1% increase in length will cause it to sag by that amount
So this is a measure of the current going through
That's the first thing, current leads to heat, changes the temperature
leads to expansion, leads to sagging.
Now I've got another magic box
This is a magic box with an N on the top because that's probably something to do with north
and something else on the bottom - south
I am not meant to take this out. George tells me that if I take it out
it will then suck itself into anything and cause great dammage
oh it's a magnet, allright, here it is
another one of these neodymium magnets
it's very, very strong and I am very clumsy with these things
you put near a piece of metal it will disappear into it.
So now I've got a magnet
if I put this horizontally nothing much happens
but if I put it this way
as I move the magnet towards the wire
you can see it's repelled quite strongly
If I turn it rount the other way
so now the north pole is at the bottom
it should pull in the other way, can you see that Brady?
yeah
it comes and hits the magnet
What's going on?
And if I put it horizontally not much happens, well
there is a magnetic field going this way
and when a current goes down the wire this way
that leads to a force on the wire
in a ... not in this direction, not in that direction but in the third direction, perpendicular to it.
And if I turn the magnetic field round I change the direction of the force
so this is the force between a moving charge and a magnetic field
which acts in a funny, funny way. Which you normally can't see
but here it's very graphic cause I got such a strong magnetic field you can actually see it beeing repelled
If I were to be a magician in the 19th century (8, 20th century?)
and I wanted to go on stage and fool people
and I set this up it would look as though I had magic powers, pushing it away
oh, float my beauty
If I now turn the current off
you have a wire without any current
and you have no effect at all, it is proportional to the current
this is sagging a little bit
it would move if it could but it doesn't want to because there's no current.
Pretty cool. Yeah I love this because I mean
thermal expansion used to be one experiment and you could never measure it
and this thing, the force on a moving current, you could never see it, it's so weak
and now I can see both, cause the magnet is so big
So this is, this is, George again to the rescue, this is wonderful, thank you George.
We're talking about the rotation of the earth or other objects and the effects of small forces that can disturb that