Patent ID: 8480294

Claim:
A balance spring comprising: a first hairspring a curve of which extends in a first plane, a second hairspring a curve of which extends in a second plane parallel to the first plane, and an attachment member securing one end of the curve of the first hairspring to one end of the curve of the second hairspring so as to form a dual balance spring in series, wherein the curve of the first hairspring and the curve of the second hairspring each include a continuously variable pitch and are symmetrical relative to a straight line parallel to the first and second planes and passing through a median plane of projection of the attachment member and wherein each order n of a balance spring moment {right arrow over (P)}( n ) is zero, where: P → ( n ) = n + 1 L n + 1 ⁢ ∫ 0 L ⁢ ⁢ ⅆ s · s n · x → ⁡ ( s ) where: {right arrow over (P)} (n) in the n th order balance spring moment; L is a length of the balance spring; s n represents a curvilinear abscissa along the balance spring to the power of n; {right arrow over (x)}(s) is a parameterization of the balance spring by the curvilinear abscissa thereof.