hairspring
/ˈhɛrˌsprɪŋ/
noun
- A fine, coiled spring in a mechanical watch or clock that regulates the balance wheel's motion.
- The hairspring is one of the most delicate parts inside a mechanical watch.
- A tiny speck of dust on the hairspring can make a watch run fast or slow.
- The watchmaker carefully replaced the broken hairspring to restore the timepiece's accuracy.