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I've always been fascinated by Breitling's circular slide rules. They look like an on-board calculator, and are a distinctive design feature, that makes the dial of the watch feel a bit more technical and “complicated”.
Macro shot of a part of the slide rule
This slide rule, called “Type 52” (since it first appeared on Navitimers in 1952) is a simplistic version of the navigation instrument called E6B (named after its part number), and corroborates the links of the watch to the aviation heritage.
This spin wheel is able to perform simple multiplications and divisions: put one number in front of another and see the result. It can also natively perform conversions from nautical miles, statute miles and kilometers. A complete instruction manual is included with the watch's papers; chances are you'll also find many web sites describing how to use a slide rule.
Before the advent of electronic calculators, such instruments were used extensively. For instance, the famous Concorde airplane was partly designed with it.
Prior to the B-1, the Breitling Chronospace (a quite resembling watch, with an Aerospace movement and a slide rule) could also calculate and convert more units thanks to its markings: LBS, IMP GAL, FT, US GAL, IN OZ, FL OZ, KG, LT. On the other hand, its slide rule was “inverted” (as compared to a conventional Type 52), making divisions instead of multiplications and vice versa.
The B-1 has the peculiarity of being equipped with an internal gearing mechanism. That way, the rotating bezel is not directly connected to the inner slide rule sleeve (unlike with most Navitimer models) and does not need large O-rings nor gaskets to help maintain an acceptable water resistance (5 ATM in this case). In addition, this does make the slide rule easier (and smoother) to use.
Schema of the gearing inside the Navitimer 1806. (Thanks to Gav and Ron)
In fact, this is an overdrive rack-and-pinion mechanism, which makes the slide rule rotate faster than the bezel (with a 1.2× ratio if I'm not mistaken). If your browser supports advanced SVG animations, you can even try for yourself with the B-1 simulation.
Because of this ratio difference, on the B-1 it's a bit harder to align the numbers and center the bezel at the same time. Still, you can align the numbers and the bezel if you keep rotating the bezel in one direction, several cycles, until everything aligns gracefully. Yes, it's just a matter of aesthetics.
Bezel centered and slide rule aligned.
Once you have found this position, even if you often use the slide rule, it's likely that you will never have to spend several minutes to find it again.
This mechanism is covered by a patent from 1967 (the famous “DDE BR 11525/67” seen on the casebacks of some Cosmonautes). Some other Breitling models in 2006 share this feature, like the B-2, the Airwolf and the Breitling for Bentley series.
You'll notice there is a hole at 10 o'clock on the left side of the case. It is not a helium release valve (unlike in a couple of high-end diver's Breitlings like the SuperOcean). It is said to be an access point used for oiling and maintaining the gears.
Gearing hole on the left side of the case.