The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com For Sale:  Special Self-Cooled Finned Spark Gaps 12 Self-Cooling Adjustable Spark Gaps - .1875" diameter Tungsten.  Perfect for lower power Tesla Coils and Ignition Coil / Induction Coil Powered Tesla Coils.  $125 each. Gap is composed of two anodized aluminum heat sinks mounted in 6061-T6 square aluminum clamps. Adjustable rod is threaded 5/16-40.  Mounted to 1/2" phenolic block. Left:  Adjustable gap.  Right:  Fixed Gap For permanent installations, fixed gaps are preferred.  Not only can you "set it and forget it" - it's also less expensive to manufacture.    For medium and high powered coils, series gaps are used.  This is a 3-series 1/4" tungsten spark gap. Cost, $175.00. Details Here is the gap in operation. This is a classic 1920s style Tesla Coil.  The secondary coil is only 4.5" in diameter and has only 250 turns of wire. At 1/2 KW (9kV 60mA transformer used here) the discharge exceeds 12".  The coil can operate for extended periods with this gap. A shot of the coil in the dark...and me in the corner! Discharge closeup. Flash photo of the setup.  3 doorknob caps - .006 mfd at 40kV, 9kV 60mA transformer, copper ribbon primary and silk-covered wire secondary coil.  6" copper float topload, with a brass bell and ball for a break-out point.