Thomas Jefferson invented the Jefferson Wheel Cypher sometime between 1790 and 1793, while he was serving as secretary of state to George Washington. The device consists of an iron rod or spindle 1/8 to 1/4 inch in diameter and 6 to 8 inches long with 36 wooden disks about 2 inches in diameter and 1/6 inch thick. Each disk has a hole drilled through its center the same size as the rod so that all of the disks can be placed snugly onto the rod, forming a wooden cylinder. The flat faces of the disks touch one another, with the outside rounded edges visible. One end of the rod has a head, like a nail head. The other end has screw threads so that a nut can be screwed onto the rod, holding the disks firmly in place.
The disks are numbered from 1 to 36 on their flat sides. The round outer edge is divided into 26 equal sections. The 26 letters of the alphabet are written or incised into these 26 sections in some scrambled order, which is different for each disk. The order of the disks on the spindle is the key for the cipher, which is nowadays called a multiplex cipher.
Here is a reproduction of a 26-disk Jefferson Cypher Wheel displayed at the National Cryptologic Museum in Fort Meade, Maryland. (Photo courtesy of Daderot under the Creative Commons CC0 1.0 license.)
