This 10-key calculator provided accurate manual computation. Its operator was called a computor. Each complete forward turn of the large crank on the right will add the value set into the 8 x 10 keys into the bottom register of the carriage. The main register on the carriage shows the result. The smaller register above it counts the turns.
Subtraction is achieved by turning the crank in reverse.
To multiply the Repeat button is pressed and the crank turned as often as needed for the low-order digit. Then the carriage is moved to the right with the handle in front, so the next digit of the factor can be cranked in. The crank on the carriage is for resetting result and counter registers. Division is performed by subtracting the divisor left to right. Courtesy of Gio Wiederhold.