Jefferson disk
Thirty-six wooden disks on an axle, each bearing a scrambled alphabet. Invented by a future president, forgotten, and reinvented a century later.
In the 1790s Thomas Jefferson described a device of 36 disks, each with the alphabet in its own order around the rim. The order of the disks on the axle is the key. The sender rotates them until the plaintext appears along one row, then copies down any other row; the recipient sets up the ciphertext and scans for the one row that reads as language.
Jefferson's sketch sat among his papers and changed nothing. The Frenchman Étienne Bazeries reinvented the principle in 1891, and in 1922 the US Army adopted it as the M-94, with 25 aluminium disks, in service until the early 1940s. The design is polyalphabetic but periodic, and with known plaintext the disk order can be reconstructed, the same family of weakness that later undid the rotor machines.