The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com Micrometer Spark Gap with Positive Stopping Positions Micrometer Spark Gap with Positive Stops: 25 positions for .001" increments (1/1000th of an inch); 160 positions for .00015625" (1.5625/10,000th of an inch) 5/16-40 TPI thread pitch Made from scrap brass (C) Jeff Behary, 2010