Characteristics of a calculator
A mechanical calculator is provided with :
The carry is automatic for the accumulator. The ten carry mechanism increments of one unit the figure located at the left of a numeral wheel passing from 9 to 0. If we except the primitive calculators, the counter is also provided with a carry mechanism.
The operating mode of a calculator depends on the characteristics of the carry mechanism: if the carry mechanism is reversible a subtraction is done by revolving the driving shaft in the opposite direction whereas in the case of irreversible mechanism , it is necessary to apply the "complement to 9" method or, as in the case of Pascaline, to use a double graduation of the numeral wheels (clockwise and counterclockwise).
The left photography shows a counter without carry mechanism. Red and white figures are printed clockwise and counterclockwise on the numeral wheels in order to facilitate the substraction and allow performing of the division.
The red figures give the number of addition cycles, the red one the number of substraction cycles.
The user must take care of the carry each time the number of cycles becomes greater than 10.