Acts like the pendulum of a watch. It is the concentric ring of metal in an escapement that extends and contracts at regular intervals. To achieve ultimate precision, the spring needs to achieve perfect isochronism. Simply put, the expansion and contraction of the balance spring needs to occur at a highly precise rate, with perfect all-round geometry, regardless of the position of the watch.
The energy source of a mechanical watch. It is a flat, cylindrical housing that has a tiny spring inside known as the ‘mainspring’, which coils up and accumulates enegery when one winds up the watch.