Maj. Albert J. Myer, a U.S. Army Surgeon, created this cipher disk to further protect Union Army communications. This disc functioned by aligning the letters on the upper two disks with the numbers on the two lower disks. In this image the letter “A” would be sent as 81. The No. line would be completed with a number to help account for the device.
National Archives, Records of the Office of the Chief Signal Officer
From The Civil War Parlor on Tumblr