Spiral spring for timepiece resonator mechanism provided with means for setting the effective length of the spiral spring

A spiral spring for a timepiece resonator mechanism, the spiral spring extending substantially in a plane and comprising a flexible blade wound on itself in several turns and setting means for modifying the effective length of the wound blade, the setting means comprising a clamp movable along at least one ending portion of the wound blade, the clamp extending substantially in the plane of the spiral spring, the clamp comprising two bearing faces arranged on either side of the wound blade in a multiplicity of positions so as to define a variable effective length of the spiral spring, wherein the clamp and the wound blade are linked to each other by a flexible element allowing the clamp and the bearing faces to be displaced along the wound blade,

• wherein the bearing faces are configured to bear on either side of the ending portion,
• wherein the clamp comprises two branches extending on either side of the ending portion of the wound blade, the end of each branch including one of said bearing faces, and
• wherein the flexible element comprises at least two flexible blades linked to the branches, and to the ending portion of the wound blade.