UvA Computer Museum catalogue nr 03.43
This Unitas Arithmometer was manufactured, probably around 1910, by Ludwig Spitz in Berlin. Both its outside (cast iron and bronze) and inside have a 19th century appearance. The machine is 67 cm wide, its weight is 19 kg.
There are two movable accumulator registers which can be operated either separately or simultaneously. In the stationary number entry register, a decimal figure (say 6) is set by operating a lever, which shifts the corresponding accumulator cogwheel with respect to a 'stepped drum' (*) such that the drum presents 6 of its 10 teeth to the cogwheel. On a full turn of the hand crank which drives the stepped-drum shaft, the accumulator wheel is rotated to position 6. Turning the crank once again, the accumulator wheel ends up in position 2, while turning its left neighbour to 1 as it passes through the 0 position.
Mainly because of the room taken by the stepped drums, machines based on this technique were larger and heavier than machines using 'pinwheels' (wheels with retractable cogs) for the same purpose, like the Odhner and Brunsviga calculators. However because of the relative simplicity and robustness, stepped-drum calculators are generally still operable today, unlike many pinwheel calculators of the same age.
(*) Stepped drum: see the lower picture, taken from the Dutch ENSIE encyclopedia (1946). The construction to the right allows either addition/multiplication or subtraction/dividing, always turning the crank in clockwise direction. A similar mechanism is used in the Unitas machine.