Complications  /  Balance spring

Balance spring

Also known as — Hairspring, spiral spring

A fine spiral spring that gives the balance wheel a natural rhythm of its own.

How it works

Before 1675 the balance of a watch had no rhythm of its own. It swung as hard as the mainspring pushed it, so any change in the spring’s force changed the rate, and watches drifted by many minutes a day.

THE BALANCEA wheel swinging back and forth,five to ten times a second.THE BALANCE SPRINGThe fine coil at its centre,tightening and opening withevery swing.
The balance spring never appears on the dial. It can be seen through a glass caseback: a wheel swinging back and forth, and at its centre a fine coil tightening and opening.
COLLETHolds the inner end on the balance staff.It turns with the balance.STUDHolds the outer end to the watch’sframe. It never moves.THE SPIRALSwing the balance one way and the coiltightens; swing it back and it opens.Either way it pulls the balance back torest, so every swing takes the same time.
The spiral links the balance to the frame of the watch. The further the balance swings, the harder the spring pulls it back, which is what keeps every swing the same length of time.

Christiaan Huygens’s spiral gave the balance a natural period, as a pendulum has in a clock. Accuracy improved from a quarter of an hour a day to a few minutes, and watches gained minute hands for the first time as a matter of course.

What it traded away is stability in temperature: steel springs weaken as they warm, and three centuries of work, from compensated balances to modern alloys and silicon, has gone into correcting that. Every mechanical watch still has one.

Recorded here

Milestones with this mechanism

1675The first balance springChristiaan Huygens fitted a spiral spring to the balance wheel in 1675, turning the watch from a rough guide into a timekeeper good to a few minutes a day.