I have been researching watch cases manufactured by the Swiss company Taubert & Fils, successors to the watch making company of François Borgel. They made a line of patented watch cases with screw back that are distinguished by having 10 flats (decagonal) for gripping with a key or wrench to unscrew them.
Some of these cases also have an inner cover, as shown in the picture of a West End Watch Co. "Sowar Prima" below. They are beautifully engineered - they fit very snugly onto a spigot in the case back inside the groove that carries the lead gasket. The little raised area is not an accidental dent, it indicates the presence of a small slot on the edge of the cover for a finger nail or screwdriver to remove it. I think the little nub in the centre of the inner cover is for the case back to hold it in place when screwed on, although there is no corresponding mark on the case back. Not all Taubert decagonal cases have these inner covers. They are sometimes described as a dust cover, sometimes as an anti-magnetic movement cover. I am puzzled by this cover - here are my thoughts on these two possibilities.
- Dust cover: The screw back, precision made and tightening down onto a lead washer, forms a very effective seal, so why would an inner dust cover be required? But if it was needed for some reason, wouldn't all such cases be fitted with it?
- Anti-magnetic cover: I have in the back of my mind that an anti-magnetic shield needs to be made of a ferromagnetic material which can divert the lines of magnetic flux around the object contained within it. This is different to a Faraday cage which can be made of any conducting material, but does not protect contained items from static magnetic fields. I thought at first that the inner cover in the picture was nickel from its colour and general appearance, which would do the job because nickel is one of the few metals apart from iron that is ferromagnetic. But it is definitely not ferromagnetic - a magnet shows no sign of being attracted to it whatsover. The stainless steel case back is actually more magnetic than the inner cover!
Does anyone know what this inner cover is actually there for? I could well be wrong about the anti-magnetic shield business, as this does appear to be the most likely reason for the inner cover. It just looks and feels more like an anti-magnetic shield than a dust cover, and you could have knocked me over with a feather when the magnet just slipped off it.
Thanks for sharing your knowledge, or for any thoughts or suggestions!
Regards - David