Hahn Calculator
In 1774, Hahn designed a truly usable calculating machine for all four types of calculation. He employed stepped drums, which Leibniz had used before, arranging them in a circular manner so that his machine appears to be similar externally to Leupold's. The machine gave complete satisfaction up to values having 12 digits. It is believed by some that Thomas de Colmar used Hahn's machine as a prototype some 50 years later and created his Arithmometer based on it. The Thomas has several main characteristics in common with the Hahn. It has not been established how many models of this machine were produced.