The Turn Of The Century Electrotherapy Museum

Dividing Head Tungsten Spark Gap
Adjustable from 1/1200th of an inch - 1/14400th of an inch!

This wheel acts like a multi-position selector switch

1 / (40 tpi) x 30 holes = 1 / 1200" increments
1 / (40 tpi) x 120 holes = 1 / 4800" increments
1 / (40 tpi) x 360 holes = 1 / 14400" increments

The disk is .050" thick.

The brass ball rides in the grooves as the gap rotates by the square key.  The gap moves 1/4800th of an inch on this setting.

The ball can be removed for the finest setting of 360 holes - the gap moves 1 / 14,400th of an inch in this instance!

This sliding arm can be locked into position for either of the 3 circular hole patterns.
Many thanks to Brian Foley for odds and ends used in this gap - like the hexagonal standoffs and this 6-32 thumb screw.

This wasn't made on a CNC!  It was manually made on a 6" Rotary Index Table from 1948!

Bottom plate.

Tungsten gap

(C) Jeff Behary, 2010