Bulle Nº 17318

At first glance this clock is a standard Bulle cased movement. But on closer inspection we can see an addition to the right sight of the movement. This is attached to the movement plate and the side wall of the wooden clock case where it terminates in some sort of engraved dial.

This is in fact an Isochron spring adjusting cam. One end of the Isochron spring is attached to the pendulum in the normal way while the other is attached to a fulcrumed arm on the adjuster. By turning the knurled arm on the adjster the spring can be tensioned very finely using the graduations on the case. A much better way of tensioning the spring correctly.