The pascaline carry mechanism
The Blaise Pascal calculator consists of identical mechanical stages corresponding to the various unit orders. This stages are connected by the carry mechanism (the "sautoir"). Each time the numeral wheel accomplishes a full turn, the numeral wheel located at his left is incremented by one unit. This sautoir is a weight which raised by the rotation of the wheels accumulates energy. This energy is released at the favourable time to pass the carry.
The main caracteristic of the pascaline is the carry mechanism. The carry over is sequential. With such a system, a computer can comprise as many stages we want, this is not the case in a system of simultaneous carry (like the one of the Schickard's machine) who, due to frictions blocks mechanics when one wants to pass a simultaneous carry on several stages (see "Technicality" "the carry").
For a decimal pascaline, each input wheel has ten rays. A serie of gears connects the input wheel to the numeral drum. The numeral drum revolves identically with the input wheel (in the image above the input wheel and the numeral drum are raised for more clearness).
When the numeral drum displays "4", the "sautoir" engages with the mechanism. At this time it is gradually raised up when the display goes from "5" to "9". When the display goes from "9" to "0" it is released and increases in its fall the following wheel of one unit.
(For the monetary calculators, the first wheel comprises 12 teeth (12 Deniers = 1 Sol) the second 20 teeth (20 Sols = 1 Livre), the following wheels being decimal).
The gears located under the input and the numeral drum comprises three discs. The first one is provided on one side with stems ensuring the connection with the inscriptor. The first two discs are connected by two eccentric stems which are used to actuate the carry mechanism.
The second disc has ten stems on the side of the numeral drum. A pawl is pressed on these stems and constrains the wheel to a well defined angular position (among ten). In this way the numeral drum is stopped exactly on a figure and not in an intermediate position. The pawl prevents also the gears to turn in the opposite direction.
The third disc has ten stems connected to the numeral drum.
Animation below illustrates the operation of the sautoir. The input wheel as well as the numeral drum have been omitted. The first disc of the first stage is semi-transparent in order to view how the fork of the sautoir engages with the two eccentric stems.
The first disc not being necessary for comprehension, it was removed on the second stage. The second semi-transparent disc allows to see how the spring lever pushes the stems to pass the carry.
The movie hereafter shows the carry on a Pierre Charrier reproduction. The camera is under the calculator. When the carry was passed, the film is rewound and replayed (what explains the erratic movement of the tens).