The Turn Of The Century Electrotherapy
Dividing Head Tungsten Spark
Adjustable from 1/1200th of an inch - 1/14400th of an
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!
(C) Jeff Behary, 2010