It's Complicated Center Seconds, Small Seconds, And Mechanical Fauxtina

Before the 1940s, almost all watches had a running seconds hand located in a sub-dial, usually at 6:00. This was due to the layout of a classic watch gear train. If you want to have a seconds hand, the easiest way to do it is to take a gear in the train that turns once per minute, extend the pivot on the dial side so it goes all the way through the plate and dial, and stick a hand on it.

In virtually all watches, the gear in question is called the "fourth wheel" as it's the fourth wheel in the gear train. The mainspring barrel, which rotates and has gears on its edge, is the first wheel. This turns the center wheel (which is at the center of the movement) once per hour. The center wheel drives the third wheel, which drives the fourth wheel. The fourth wheel turns once per minute, and if you put a hand on its pivot, you've got a seconds hand. That means that if you can find the fourth wheel when you're looking at a watch gear train, you can figure out where the seconds hand should be.

Everyone ready to play "Find the fourth wheel?" Let's look at a traditional, small-seconds gear train layout: 

A classic gear train layout: the ETA 6497. The mainspring barrel is the larger of the two gears on the left, with the rest of the gear train (or going train) hidden under the bridges. The balance is to the far right, along with the balance spring.

With the bridges removed, you can see the entire going train (the balance wheel has also been removed). From left to right, the gears are: mainspring barrel, center (hour) wheel, third wheel, fourth wheel, the club-toothed escape wheel, and finally, the lever of the lever escapement. The 6497 was the last ETA caliber to be designed specifically for pocket watches, so in a pocket watch the crown would be at 12:00 (where the winding stem is) and the small seconds display at 6:00, where the fourth wheel is. (There is a version of this caliber designed for wristwatches, with the stem and crown at 3:00 instead of 12:00; that's the ETA 6498).

And the dial side. The small seconds hand is turning on the extended pivot of the fourth wheel. The minute hand is on the pivot of the center wheel (which turns once per hour), and is geared to the motion works which drive the hour hand.

As a side note, if you look at the history of hands in watchmaking you'll see something pretty cool, which is that minute hands and seconds hands appear more or less in conjunction with improvements in precision. The earliest watches generally ran for less than a day and were accurate only to the closest hour, if that, so they usually had just an hour hand. But add a balance spring and suddenly a minute hand makes sense; add improvements to escapements, and temperature compensation, and it makes sense to have a seconds hand.

The classic sub-seconds dial is an elegant and mechanically simple solution and to this day, some of the most beautiful watches use it, including those from Philippe Dufour and Roger Smith. The only disadvantage of a traditional small seconds hand is that they're hard to read, especially in smaller watches. A better solution is to put the seconds hand on the same axis as the hour and minute hands, where it can be lengthened so that its tip sweeps around the entire circumference of the dial.

There are a couple of ways to do this. The first center seconds movements to appear in any number were indirect center seconds movements, like the Zenith caliber 133 and the Patek caliber 12'''-120. Omega made quite a few indirect center seconds movements, as well. The way these movements and their ilk work, is to have a traditional gear train layout, but to add gearing which sits above the gear train and drives a seconds wheel at the center of the movement. You can drive an indirect center seconds gear off either the third or fourth wheel of the gear train.

The only problem with this is that since the additional gearing for the seconds hand is out of the direct power flow from the mainspring barrel, there is a tendency for the seconds hand to flutter. To address this, watchmakers added a tension spring that puts very slight pressure on the indirect seconds gear teeth.

This, however, creates two new problems. The first is that since the indirect seconds gearing sits on top of the main gear train, you end up with a noticeably thicker movement. The second problem is that the tension spring adds a frictional load to the gear train and if it's not adjusted exactly correctly, you get reduced balance amplitude and therefore, a loss in precision, which is ironic since a center seconds is supposed to be an aid to reading off the seconds more precisely in the first place!

For this reason, watchmakers came up with direct center seconds movements. In these movements – and virtually all modern movements use a direct center seconds design – you solve the problem by rearranging the main gear train, not by adding gears. If you tweak the gear layout so that the fourth wheel is at the center of the movement and not off to one side, then you can just run the pivot through to the dial as you would for a seconds hand in a sub-dial.

Above is the ETA caliber 2895-2. As you can see, the going train is now laid out in a row, starting at 10:00. The mainspring barrel drives the center wheel (the one closest to the edge) and moving inward, we have the third and fourth wheels, with the fourth wheel right at the center of the movement. This is a typical set-up for a movement with center seconds, and it's identical to the going train for the 2892 (also, of course, a center seconds movement).

Now, a side-note to all, this is the problem of what to do if you're a modern watch brand, and all your movements have direct center seconds layouts, but for some reason you want to have a small seconds sub-dial?

In order to do that, you have to do something that the center seconds hand was designed to do away with, which is add additional gearing. Sometimes (not always) watch brands do this when they want to duplicate the look and layout of a well-known vintage model. Often the small seconds display is handled by the addition of a module under the dial. 

Below is the dial side of the ETA 2895-2. As we said, the going train is the same as the ETA 2892, but flip the movement over and you'll see that, in the 2895, there is additional gearing to support a small seconds display.

 The going train is designed for center seconds, but a module's been added to support a small seconds subdial. This is how you get a small seconds display in, for example, the TAG Heuer Caliber 6.

Modern center seconds movements with added gearing to support a small seconds hand are fascinating. It doesn't aid legibility, of course, and it's a bit of a break with a basic principles of good engineering which is to not add complexity for no reason. But it does mean you can recreate a vintage look, without having to invest (a lot) of money in designing and tooling up for a new movement.  

You can use a center seconds movement modified to support a small seconds subdial for lots of reasons – of course one of the most common is to give the design a vintage look and feel. But not every modern watch with such a movement is specifically shouting out to the good old days, and like colored lume, it's just another choice in the very large toolbox modern watch designers have. (It's ironic that to get a small seconds in a modern movement, you often have to add gears, which is what watchmakers had to do to get center seconds from a small seconds movement in the first place, but I feel like what you get a lot of in modern horology, is irony.) Mechanical fauxtina? Maybe sometimes. But the fact that it's an option means a wider range of style and design choices for enthusiasts.

Thanks to Jonathan McWhorter at Crown & Caliber for the images of the 2895-2. For more on center seconds from the OG technical watch writer, Walt Odets, check out "The Pursuit Of Center Seconds" on Timezone.com, Part 1 and Part 2.