Small seconds
A layout that follows from how the movement is built, not an added complication
What it is
A separate small dial, usually at six o'clock or nine o'clock, carrying its own seconds hand. It shows the same seconds a centre hand would, just on its own axis so the three hands never overlap. It is a dial layout that follows from how the movement is built, not an added complication.
History
Sub-seconds is the older arrangement, and it exists for a structural reason: in a conventional going train the fourth wheel, which turns once a minute, sits off centre, so a pinion extended from it drives a seconds hand wherever that wheel happens to be. Nineteenth-century precision pocket watches and American railroad watches standardised the small-seconds register at six, and it carried into early wristwatches unchanged. Centre seconds arrived in the twentieth century, at first as an indirect drive, with an extra wheel carrying the reading to a central pipe and a friction spring fitted because the hand sat outside the flow of power and would otherwise flutter. Movements were later redesigned to put the fourth wheel in the centre so the hand is driven directly; Zenith's calibre 133 of 1948 is commonly cited as an early example of that rearrangement, though as an early instance rather than a documented first.
How it works
In a direct small-seconds movement the hand is mounted straight onto the extended pivot of the fourth wheel, which the train already turns once per minute, so nothing is added at all. An indirect centre seconds takes the same rotation through an intermediate wheel to a central seconds pinion; because that pinion is driven off the side of the train rather than in it, a friction spring is needed to damp backlash and stop the hand shivering. A direct centre seconds reorganises the train so the fourth wheel itself is central, which costs more in movement architecture but gives a steady hand. So a sub-seconds subdial is almost always direct off the fourth wheel: the most honest and mechanically cheapest way to show seconds.
Parts required
Nothing beyond the base movement's own fourth wheel and an extended pivot with a hand fitted. It is the centre-seconds versions that add an intermediate wheel, a central seconds pinion and a friction spring.
In the catalog
Related
- Gear train: The series of wheels reducing the mainspring's speed to readable time
- Hands: The moving pointers that display time
- Regulator: An observatory clock’s dial, borrowed for the wrist