Spiral with immobile mass centre

摘要

The spring (1) has an attachment member (4) securing an end of a curve of a hairspring (3) to an end of a curve of another hairspring (5) to form a dual balance spring in series, where a part of the spring is coated with silicon dioxide. The curves respectively extend in two parallel planes. The curves are symmetrical relative to a straight line parallel to the planes and passing through a median plane of projection of the attachment member. Each curve respects a specific relation to reduce displacements of center of mass of the spring during contraction and expansion of the spring.