hairspring

from WordNet (r) 3.0 (2006)

hairspring
    n 1: a fine spiral spring that regulates the movement of the
         balance wheel in a timepiece

from The Collaborative International Dictionary of English v.0.48

Hairspring \Hair"spring`\ (-spr[i^]ng`), n. (Horology)
   The slender recoil spring which regulates the motion of the
   balance in a timepiece.
   [1913 Webster]