The key to this coil's performance on such a tight budget is that all of its components are designed to work well together. Using some basic concepts from AC circuit design, the components are matched to perform well without requiring massive amounts of power. Some "coilers" use microwave oven transformers to pump kilowatts of energy through poorly matched circuits, resulting in large losses and mediocre performance. This instructable will show you how to avoid making such mistakes and how to properly design a spark gap Tesla coil.
UPDATE: This Tesla coil is now on sale on eBay for a starting bid of $99.99, less than the cost of the materials! http://www.ebay.com/itm/250-000-Volt-TESLA-COIL-Assembled-2-Foot-Tall-8-12-Sparks-/180826521311?pt=LH_DefaultDomain_0&hash=item2a1a19bedf
For contest entry details on this instructable, see step 10.
UPDATE: New diagrams for primary capacitor, primary coil, and spark gap construction have been added. Click the top left information icon to view them in full size.
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(I named it the "Valentine's Day Tesla Coil" in this video because I finished it over Valentine's Day weekend 2011)
To read more about this project, visit my website: http://xellers.wordpress.com/tesla-coils/sgtc-ii/