Index for flat balance springs is a device used to accurately set the position of the flat balance spring in a mechanical watch movement. It consists of a rotating wheel with a number of narrow slots or teeth along its circumference. The position of the index wheel is set by a lever that can be adjusted to a precise position, which in turn is used to control the tension of the balance spring. This allows for greater accuracy in timekeeping by ensuring that the balance spring is correctly tensioned.
