Adders
Adders Using Rods

A collection of adders including a Magic-Brain Calculator, an Exactus Mini-Add, a Troncet Arithmographe, and a Locke Adder.

From antiquity, mathematicians have known that one can represent numbers by lengths. To find the sum of two numbers, one can use two rulers that slide next to one another, assuming the rulers are long enough. In 1700, the Frenchman Claude Perrault suggested that one could represent each digit in numbers to be summed by a separate sliding stick. Another Frenchman, C. de Caze, actually built such an instrument.

The Smithsonian collections include several models of adders submitted to the United States Patent Office. Some of these represented numbers by the length of movable rods. Another rod-type adder, invented by Clarence E. Locke of Kensett, Iowa, was the first American-made adder to sell successfully for several years.