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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 :)

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@freemo Get your hands on a hipot tester. Can make black ink on paper sparkle.

@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.

@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?

@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.

@skobkin no, 40V but i should have said 3.2A since it is current limited and for short runs of pencil "lead" it is only running at about 5V.

@skobkin That said I have run it at high voltages of (40ish V) and it goes up almost instantly at that voltage/current. Just not what I'm running it at today.

@freemo
Yeah, I guess 3.2A will make it quite hot too ๐Ÿค”
Need to try that myself I guess.

@skobkin just use it to short a 12V car battery and it will go up pretty fast.

@freemo
I have 4 UPS batteries at my disposal, but they're perfectly balanced and I'm not sure if I should do that to them.
But I have a couple of power supplies. Can use them. Still need to buy this graphite for pencil though.

@skobkin if they are lead-acid batteries they should handle it fairly well... if you hook them together (series or parallel) they should remain balanced..

that said just short one of your outlets, lol. The worst that could happen is you burn your house down :)

@freemo

Keeping playing with it and sooner or later you'll invent the light bulb.

The first light bulb filaments were carbon. I'm sure they did exactly what you did and thought, "Gee, that lit up the whole room. If we could only keep it going."

Thus the vacuum bulb to keep oxygen from burning it up.

@Ianhorsewell

@Pat @freemo @Ianhorsewell
That's what the nerds come up with โ€ฆ

And then the business suits enforced their Adult Supervision:
"Gee, if we build vaccum bulbs as good as we can, we can't keep selling crappy ones"
en.wikipedia.org/wiki/Phoebus_

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