A balance cock is the bridge that holds the upper pivot of the balance wheel from above. It is fixed to the main plate by a single screw and located by feet. It also carries the upper shock absorber. Because it supports the balance, its stiffness and exact placement affect how the movement works.
How does a balance cock work?
The balance cock sits above the balance wheel and holds the top pivot of the balance staff. Feet on its underside fit into the main plate and set its position. A single screw then holds it down. Because it is fixed at only one point, it works as a cantilever, a beam supported at one end only. To stay rigid, it has to be short. In traditional designs the cock also carries the stud holder. The stud is a small post fixed to the cock, and the outer coil of the hairspring is anchored to it.
Where it comes from
In early full-plate pocket watches, the balance wheel sat outside the back plate. The balance cock held it there. Later, three-quarter plate movements cut away part of the back plate. This made room for the balance and its cock, and it let the watch be thinner. On antique watches and clocks the cock was often richly decorated. Makers pierced it with open patterns and engraved its surface.
Not to be confused with
A balance bridge does the same job as a balance cock, but it is held at both ends rather than at one. Some movements use a balance bridge in place of a cock because it is considered more stable. A cock, fixed at a single point, relies on being short to stay stiff. A bridge has support at each end, so it does not depend on that alone.
How is a balance cock finished?
Balance cocks are often finished with bevelling, circular graining and engraving. Bevelling shapes the edges into a sloped band. Circular graining covers a flat surface with small overlapping circles. Engraving cuts lines or patterns into the metal. The finish is there for the eye, but the cock still has a mechanical job to do. In modern watches, how rigid it is and how precisely it is positioned still matter a great deal.
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