Patent Model for an Adder with Carry by Alonzo Johnson

Johnson, Alonzo · 1868

This patent model for an adder has as its base two concentric brass discs, one rotating inside the other. The rim of the outer disc has the numbers from 0 to 99 engraved around its edge. The inner disc has one hundred small holes marked evenly around its edge. These also are numbered 0 to 99. Two steel arms pivot at the center of the disc. The longer arm has a pin on the underside that fits into the holes and a small knob on the upper side so that it can be rotated. A protruding pin set at 0 in the outer circle stops the motion of this arm. It is used to add numbers up to 99.