The carry mechanisms
The difference between a calculator and and instrument of calculation is than for a calculator the carry is automatic. A simple reduction of a factor 10 between gears is not a good solution. In this case, the numeral dials can be positioned in variable angular positions, the digits of the results are not aligned in a straight line and the reading becomes difficult.
The most simple mechanism for the carry(implemented by Schickard in his machine) was employed in the old car mileage counters.
The simultaneous carry
Each numeral dial has a single tooth mutilated gear which on each complete turn engages the neighbouring dial (an intermediate gear between two adjacent numeral dials is required to keep for all the dials the same revolving direction).
This kind of carryforward is known as 'simultaneous' because when several wheels of the adding machine are to '9' such as for example for number 999, the fact of adding a unit makes pass at the same moment three wheels from '9' to '0'. It follows that the mechanical effort requested from the first pin is proportional to the number of carry to make. In practice these machines are blocked when a carry is required on 4 or 5 adjacent wheels.
Clic on the photo to see a simultaneous carry.
The sequential carry
A solution to the problem of the simultaneous carry consists to pass the carry sequentially to the successive decimal orders. With such a solution, the effort reqired on a wheel is always equal to the effort of one carry. Various mechanical systems were developed to achieve this goal.
Clic on the photo to see a sequential carry.
The comptometer problem
In the case of the full keyboard adding machines like the ' Comptometer', a technical difficulty has to be solved. The fact of being able to enter at the same time several digits of a number requires to be able to perform a carry on a wheel which is moving. It is necessary in this case to give to this wheel a relative profit of a tooth.
The adopted principle consists in, when a numeral wheel is moving, accumulating energy in a spring. This spring is interdependent of a gear maintained at rest as long as the numeral wheel does not pass from 9 to 0. At this time, the gear is released and the carry passes to the neighbouring wheel (more details at the "John Wolff's Web Museum").