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I love running 40V across 0.5mm pencil lead and watching it evaporate into gas in a flash of blinding white light :)
@freemo wire wool and a 9V cell is a nice demo which sets the scene for fuses. Although it also teaches kids how to start fires, McGyver-style.
@Ianhorsewell a classic indeed.
The thing i find fun about the pencil lead is just how insanely bright it gets... it will cause eye damage if you look at it.
@Ianhorsewell While I have never tested this, I would expect there to be yes...
When the carbon gets hot it starts to react with the atmosphere producing CO2 as gas (though no noticeable flame) this causes the graphite to thin out. As it gets thinner its resistance goes up and it gets hotter, and brighter. As such it will start a dull red and slowly thin out as it gets to a blinding white light, at the end it breaks apart in a final flash of bright white light and then the circuit is broken.
I would expect in a vacuum the carbon would remain unreacted and likely wouldnt have the same runaway reaction.
@freemo I feel like I should have played with this more back when I had a school lab instead of a laptop. Is there an observable difference if you do it in a vacuum bell jar or N2 atmosphere?