The Omnigraph model shown here consists of a variable speed spring-powered motor and a stack of aluminum disks which have patterns of dots and dashes "encoded" in their serrated edges.  As the motor runs, the disks rotate, a "finger" on a leaf spring follows the pattern on the disks, and operates a set of contacts. 

The basic Omnigraph model used only a single code disk.  This severely limited the length of the recorded code pattern, and students could cheat once they memorized what was on the disk.  More expensive models solved this problem by using multiple disks and a mechanism to move the contact finger up and down to read the code patterns from each of the disks.  Models using five or fifteen disks were available.  The fifteen disk model is shown here, and was known as the "No. 2 Omnigraph". 

There were deluxe models that had electric motors, and some that were hand cranked.  Some had telegraph sounders and keys mounted on the board.  A number of different sets of disks were available, and they could be had either in Continental or American Morse code variants.

Extending outward from beneath the stack of disks are five metal rods.  As the stack rotates, each of these rods moves beneath a star wheel.  As each rod goes by, it rotates the wheel by a fraction of a turn.  A set of gears transfers this motion to a heart-shaped cam, which moves the contact finger up and down to sample each of the 15 disks.  The stack of disks turns counter-clockwise, when viewed from above.  In the above picture, the rod at the right is just about to rotate the star wheel as it moves toward the back.  The rod in the foreground will be the next to act as it moves to the right.

To make things more interesting, any of the five rods can be individually retracted so they don't contact the star wheel as they go by.  When all five are retracted, the contact finger stays on the same disk for as long as the motor runs.  If one rod is extended, each disk is "played" in its entirety before moving on to the next disk.  If all five are extended, only one-fifth of each disk is played before moving to the next.  As the sequencing continues, all segments of each disk will eventually be played.  Each of the disk segments contains 6 or 7 characters.

The sequencing can also be altered by stacking the disks in different orders, and in different orientations with respect to each other.  And, of course, there were alternate sets of disks that could be used.  

This simple, but ingenious scheme allowed the instructor to vary the patterns in nearly limitless numbers of ways, far beyond the ability of students to memorize.