In the views on this page, the rotor and carriage are only partially assembled so as to show the principles of operation more clearly.

The basic addition mechanism is very simple. As the rotor is turned, the extended pins on the pin-wheels act as gear teeth to advance the intermediate star wheels, which advance the numeral wheels by the same number of positions.

The numeral wheels are held in position by detent levers, which are spring-loaded by coil springs and steel balls in the cylindrical wells. The vertical arm on the detent lever acts as an escapement to prevent over-run on the final step.

This view from the left rear of the carriage shows the arrangement of the carry sense levers. The key features are the operating finger at the top front of the lever, the offset ramps and tongue at the top rear, and the tail and detent pin at the bottom.

The lever is pivoted on a rod at the rear of the carriage, but its movement is restrained by the intermediate star wheel shaft which passes through the slot at the top. The forward-facing tail on the bottom of the lever acts against a spring-loaded detent pin in the base of the carriage, and holds the head of the lever securely in either the forward or rearward position.

A machine cycle starts with the carry sense levers reset in the forward position (as shown). If the display side of the numeral wheel passes between 0 and 9 in either direction, the finger between digits 2 and 3 will toggle the sense lever into the rearward position.

After the addition pins have passed the star wheels, the carry fingers sweep past in sequence from right to left as the rotor turns clockwise (or reverse for subtraction).

If the addition has not resulted in a carry, the sense lever will remain in the forward position. The fingers on the rotor disc will pass clear of the offset extensions at the back of the sense lever, and nothing will happen.

If a carry is required, the sensing lever will have been pushed (and latched) rearwards.

As the rotor turns, the spring-loaded carry finger on the next rotor disc will meet the ramped surface formed on the back of the carry sense lever. The finger will be pushed sideways, moving it to the left and into the path of the next star wheel (not shown). The finger will advance the star wheel by one position as it passes.

The sense lever remains in the rearward position, with the offset tongue behind the ramps resting in the groove between the two rotor discs.

This view of the under or right-hand side of a pin-wheel shows the outer groove in which the carry sense lever rests. The groove is shaped into a cam, which resets the sense lever by pushing it forwards near the end of the cycle.

This detail view from the front of the machine shows the alignment and sequencing of the add-and-carry mechanism when fully assembled.

Note the arrangement of the carry fingers in sequence from right to left across the rotor. This view shows the carry phase of the cycle about to commence on digit 2 (from the right), although digits 4 and 8 have not yet completed their additions. (Naturally, there is no carry mechanism on digit 1).

On digits 2 and 3 the carry sense levers are shown rearwards, with their reset tongues in the grooves between the rotor discs. Digit 2 clearly shows how the carry finger will be pushed sideways to advance the next star wheel.

A second set of carry fingers is arranged symmetrically on the opposite side of the rotor for use in subtraction cycles.