Abstract :
Gottfried Wilhelm Leibniz had the conceptual idea for a four-function calculator at the beginning of the 1670s. It was an extended adding machine with tens-carry, not a genuine multiplying device. He ordered the manufacture of several models, but had to leave the design work and the constructive details entirely to his co-workers. Leibniz pursued this project over four decades (1672 to 1716), but, aside from several demonstrations, first in London and later in Paris, Leibniz published very little about his machine, and even that he did very late. True details became known only as of 1887 due to W. Jordan´s publications. Prior to that, only the exterior of the machine was made public via a drawing. Leibniz was definitely the inventor of the stepped wheel, but this did not become apparent until 1892. Only Gerke, Reuleaux and Jordan had seen the stepped wheel in 1880 and 1892. A. Burkhardt documented it in great detail in 1896. Only one original Leibniz calculator still exists today at Hannover. Thus, the answer to the title question is 1892. It is evident from all we know today that Mathias Hahn (1739-1790) and Thomas de Colmer (1785-1878) each invented his own stepped wheel, independent of Leibniz
