Fun with graphite

27.08.2007 22:46

I found a video demonstrating how to use a glowing graphite pencil lead as an emergency light source. It seemed like a fun experiment to try. Unfortunately I didn't have a charged 12V car battery at hand. I started charging one and meanwhile tried the same thing with a 5V lab power supply.

The original video uses an ordinary pencil lead which has a diameter of something like 2mm. Since the power supply I was using wasn't rated for the current that would flow through such a thick rod of graphite I used a 0.7 mm pencil lead instead.

In the video you can see that even with 5V the lead glows nicely. You can also see the way it degrades: the middle of the lead is hottest because heat is conducted away from the ends by the metal clips. Since graphite will oxidize/ablate faster at higher temperatures the center part gets thinner with time. This increases the resistance of the center which means that 1) less current will flow through the lead and 2) the center starts to heat more than the ends. It takes around 40s for the center part to become so thin that it heats so much that the graphite vaporizes and breaks the circuit.

Glowing pencil lead

(Click to watch Glowing pencil lead video)

The instrument on the left shows current in Amperes (actually it shows a voltage drop across a 1mΩ resistor hence the mV mark at the top of the display). You can see that the current is decreasing as the lead degrades. Interestingly, it does not seem to fall significantly while the lead is heating up as is the case with a metal filament in an ordinary light bulb. Electrical power with a fresh lead is around 30W.

Spent graphite leads

The spent lead is shaped into a cone.

Posted by Tomaž | Categories: Life

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