Friday, June 06, 2008

Updates: Tesla coil and light bulb


I tried to help Igor out on his Tesla coil. It's a traditional spark-gap variety coil. The problem was the secondary kept arcing over to the primary or elsewhere from near the bottom, nowhere near the top, and we didn't see any streamers or such from the top. Nothing we tried would insulate the primary from the secondary--styrofoam, electrical tape, plastic cups. When we changed where we connected to the primary the sparks would change their behavior--sometimes we would get a single spark or sometimes we would get what we think was called "racing sparks"--the ones that jump a few inches on the secondary. Also we tried grounding, although we were up on a third story. All this, plus the fun of a spark gap that literally was too loud to operate. Bang-bang-bang! We need to enclose the spark gap in something to absorb the sound, but to do that would mean we'd also have to set up an air flow via a fan to break the arc. No photos of this work.

The other is a revisit to a Scav Hunt item, a homemade light bulb. This was a success on our part. We used a 0.5mm mechanical pencil lead, connected it to a pair of 6V latern batteries wired in series, giving 12V. The filament was enclosed in a Snapple bottle with wire passed through a hole in the cap sealed with hot glue. To remove oxygen in the air we lit a match or two in the bottle until it went out and sealed quickly. This bulb had a nice ruddy glow to it. We only ran it for thirty seconds or so before it was passed on for the judges to see. I don't know where it is now, so I decided to make another one. The basics were the same except for the power source. I used a 15V AC 1A power supply. I put the filament in a Starbucks glass container that I had worries about being airtight enough. This time it lit nicely, but within ten seconds one side of the graphite became brighter. A post-mortem showed it got brighter because the lead got narrower, and more power was dissipated there, increasing the erosion. It got brighter and brighter and then narrowed to nothing, when it broke.

Graphite filament assembly

Light bulb lit

Lightbulb movie -- Click to play -- sorry for the poor aspect ratio

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