The active length of the hairspring is from the pinning point at the collet to the counting point or index pins. The active length of the hairspring is altered by the movement of the curb pins along the end curve. Simply put, a shorter hairspring takes less time to return to complete one full oscillation than a longer hairspring. The position of the curb pins denotes the active end of the hairspring, effectively varying its length. By making the spring shorter (by moving the curb pins closer to the two 45 degree bends in the hairspring), the timing speeds up, and similarly by making the spring longer (by moving the curb pins closer to the stud), the timing slows down.
Also known as a ‘terminal curve’, the end curve follows the two 45 degree bends in a hairspring that enable the hairspring to pass cleanly through the curb pins. The end curve connects to the stud and the movement of the curb pins along the end curve alter the ‘active length’ of the hairspring, which is how the watch is regulated. By making the spring shorter, the timing speeds up, and similarly by making the spring longer (by moving the curb pins closer to the stud), the timing slows down.
The end curve can also refer to the bend in the centre of the hairspring where it attaches to a collet.
Index pins restrict the active length of the hairspring. By making the spring shorter (by moving the index pins towards the two 45 degree bends in the hairspring, that mark the start of the end, or terminal, curve), the timing speeds up, and similarly by making the spring longer (by moving the curb pins closer to the stud), the timing slows down.
The result of wide index pins will cause a loss at low amplitude (curb pin error) as the spring will not have enough power to touch either of the curb pins and thus take its timing point from the stud, which increases the active length of the hairspring making the watch run slow.
The travel of the lever from the banking pin to the point when the guard pin makes contact with the safety roller.
This can occur during the supplementary arc when the watch receives a shock.
