Above is the JLC caliber 849. This is one of the classic ultra thin movements of the 20th century, and at 1.85 mm thick, it’s an excellent one to look at in terms of why making thin movements is so challenging. There are a number of reasons it’s harder to make a very thin watch, and make it work well, than it is to make a thicker watch. First of all, there’s the question of power. If you want a watch to run accurately, it’s essential to have adequate power fed through the gear train to the balance wheel, so that it can oscillate at a large enough amplitude (amplitude just means how much the balance swings, and is generally expressed in degrees) to maintain accuracy across the working power reserve of the watch. The amount of power you can get from a mainspring is generally dependent on the height of the spring, and of course, in an ultra thin movement there is considerably less height available. That means that the movement has to be made and assembled very carefully and precisely, in order to avoid excessive loss of power due to friction.
Another reason it’s challenging to make a very thin movement is that as a rule, you have to make significant changes in movement architecture. As an example, in most watch movements, the mainspring barrel has two pivots – one running in a bearing in the mainplate, and the other in a bearing in the barrel bridge. The JLC caliber 849 has what’s called a “hanging” barrel (which was originally invented by Lépine, who you’ll remember from Part 1 of this story). A hanging mainspring barrel has no upper bridge; instead, it runs only in one pivot: the one in the mainplate. The construction is inherently less stable and again, requires a lot of care in manufacturing and design to work well, but getting rid of the upper barrel bridge saves precious millimeters and is essential, in the 849, in bringing in the height at under 2 mm.