Hairspring with two spiral springs with improved isochronism

The spring (1) has hairsprings (3, 5) comprising curves extending in planes, where each curve has continuously variable pitch. The curves are symmetrical relative to a straight line (A) parallel to the planes, and pass through a median projection plane (P) of an attachment element (4). Each curve is arranged such that specific relation is zero to reduce displacements of center of mass during contraction and expansion. Each hairspring has H-shaped counterweights (8, 8', 9, 9') to compensate for unbalance formed by a mass of the element and personalize anisochronism slope of the balance spring. The specific relation includes a moment of the balance spring of order n, length of the balance spring and curvilinear abscissa along the balance spring to the power of n and parameterization of the balance spring by the abscissa. The balance spring is formed from silicon and includes a part coated with silicon dioxide.