Graphite filament
This demo shows how how you can produce light by heating something up electrically in a reliable and impressive way.
This is a classic, simple demo that is surprisingly hard to do, because it is hard to hold a very brittle, thin propelling pencil lead without it breaking.
This setup is machined with a groove between the two stainless steel pillars that is completely in line, so that there are no bending forces on the lead, so it reliably doesn't break.
It can also be built in a vacuum jar so that you can show how much removing oxygen helps the life of a filament.
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Demonstration