The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com Micrometer Spark Gap with Positive Stopping Positions 1 / 10,000th of an inch increments This gap starts where the other left off! 5/16" - 40 TPI threaded assembly in combination with a machined wheel to indicate positions along the thread. 25 holes located 1/1000th of an inch positions, and 250 slots mark 1/10,000th of an inch positions. (1 / 40*25 and 1 / 40*250) Top view of spark gap.  Main knob is made of Ebony wood - made from a stringed instrument key. ( http://www.lvstrings.com ) The second key to the right is rosewood, and is used for the incremental settings by 1 / 1000th of an inch. Both Rosewood and Ebony turn nice on the metal lathe even using normal carbide tipped tools. To set the gap for .005", remove the pin and rotate the wheel to the .005" position.  .  . .  .  .then insert the pin, locking the mechanism securely in place. A close photo of the gap setting at .005" To set the gap to .0053", remove the pin and place to the side.  Use punch mark to align the 3rd notch past .005". Template used to make the wheel Io Sumo (C) Jeff Behary, 2010