The balance-spring is formed by a first assembly, comprising the coils (3) and the outer curve (2), made in a first material such as Elinvar having an elastic torque that is insensitive or almost insensitive to elongation, temperature and magnetic field variations, and by a second assembly secured to the first assembly, comprising in particular the inner curve (4) made of a second material essentially selected for its mechanical properties and for the shapeability of said inner curve (4) along the most favourable contour for concentric expansion of the balance-spring, such as a Grossmann curve (14), said second assembly also being able to comprise a collet (5).

The invention relates mainly to a hairspring intended to equip the balance wheel of a mechanical timepiece. It is in the form of a spiraled rod (10) cut from an {001} single-crystal silicon plate. Its structure and its dimensions are calculated so as to minimize the thermal drift of the hairspring/balance-wheel assembly by means of the first and second thermal coefficients of its spring constant.

A spiral spring for use in connection with horology instruments is disclosed which includes a spiral spring and a collet which are produced in one piece without molecular discontinuity.

A mechanical timepiece movement has a mainspring serving as a power source, a wheel train rotated by a rotational force produced by the mainspring for driving a display, and an escapement and speed control apparatus for controlling rotation of the wheel train and having a balance with hairspring, an escape wheel and pinion and a pallet fork. A stud holder is rotatably mounted with respect to a central axis of the balance with hairspring and a hairspring holder is attached to the stud holder to undergo movement in a direction orthogonal to the central axis of the balance with hairspring. A hairspring has a central portion fixed to the balance with hairspring and an outer portion fixed to the hairspring holder. The hairspring is adjustable by pivoting the hairspring holder.