Holman Adding Machine
From the collection of National Museum of American History. Free to download, adapt and use — no permission needed.
This round metal patent model for an adder with carry has a cylindrical case that contains two concentric rotating rings, each with the digits from 0 to 99 engraved evenly around the edge. The outer, lower ring, with digits representing hundreds, is slightly larger than the other ring. Atop these is a metal disc with two adjacent windows, which make it possible to see the digits on the discs below. On top of this disc is a flat ring with four spokes that hold it together. This ring is the size of the lower ring. This ring also has 100 divisions around the outside, which are engraved with numbers. There is a window at 1 that allows one to see the disc below. This spoke has a small knob for rotating it.
National Museum of American History’s own description
