A disc for a Morse code training device. Projections around its edge encode dots and dashes. As the disc rotates, the projections open and close an electrical contact causing the sounder to click. This example, the No. 2 Omnigraph, holds 15 discs and cost $30. There was also a cheaper version that held 5 discs. The Omnigraph Manufacturing Co. was in business from c. 1900 to c. 1931 (Shultz 2023).

The challenge for the designer was to make the sequences of letters as unpredictable as possible—otherwise the discs would only be useful untilww the student memorized the sequence. Each of the 15 discs has 5 independent sections. Corresponding to each section is a metal rod projecting radially from beneath the discs. As the disc turns, the rods engage with a mechanism that raises the electrical contact to the next disc in the stack. One or more of the metal rods can be pushed aside by the operator to add even more unpredictability. The discs can also be restacked in a different order.