hairspring

If this is a balance pivot, be very careful to see that your balance is true and poised before putting on hairspring and roller.
Scientific American Supplement, No. 664, September 22,1888|Various
The hairspring stud E is of steel, shaped as shown, and comes outlined with the other parts.
Watch and Clock Escapements|Anonymous
We refer to arranging and disposing of the impulse of the escape wheel to meet the resistance of the hairspring.
Watch and Clock Escapements|Anonymous
As we set the bankings the pallets are all right; but to test matters, let us remove the hairspring and put the balance in place.
Watch and Clock Escapements|Anonymous

The Curb Pins sometimes produce the ferric oxide mentioned by their action on the hairspring.
Friction, Lubrication and the Lubricants in Horology|William T. Lewis

British Dictionary definitions for hairspring

hairspring

/ (ˈhɛəˌsprɪŋ) /

noun

horology a very fine spiral spring in some timepieces, which, in combination with the balance wheel, controls the timekeeping

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012