The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com Micrometer Spark Gap Inspired By Thomas Burton Kinraide Adjustable in .0003571" Increments! Adjustment screw is threaded 5/16" - 40 TPI.  Knurled nuts have 70 grooves.  1 / (40 x 70) = 1 / 2800 = .0003571" per click! This thumb screw can rotate the tungsten contact in case of uneven wear Internal mechanism showing rotating shaft. Post alignment adjustment knob and fixed contact. Ratchet mechanism. Finally hung the small gate!  Cip did an amazing job on this!