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• 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

• 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

This is two amp fuse wire

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B1 wire current fuse magnet magnetic field george

# Current and Magnets - Sixty Symbols

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林宜悉 posted on 2020/03/30
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