Balance wheel

The balance wheel is the weighted wheel in a mechanical movement that swings back and forth at a steady rate, working with the hairspring to divide time into equal beats.

It's the heart of the movement's accuracy. Damage from a knock, magnetism affecting the hairspring, or worn pivots show up as poor timekeeping.

A timegrapher reading of the balance's amplitude and rate is the quickest way to judge whether a pre-owned watch's regulating organ is healthy or needs a service.