This changed with Huygens' invention of the pendulum. This coupled with Clement's recoil escapement produced a clock that kept time to a few seconds a week.  The portable timekeeper, on the other hand, was still flopping around with miserable results.

The problem had been known for a long time.  In the rennaisance, German watchmakers tried to make the balance less dependent on train variations by controlling it's outer oscillations with a springy hog bristle. The length of the bristle could roughly control the speed of the balance.  Didn't work much at all, but the idea had been floating around.  After the pendulum raised the stakes, a lot of makers tried to experiment with flat springs as a substitute for the hog bristle, with hardly better results, but a lot of folks thought that their efforts gave them a shot at credit when the coiled balance spring came into use.

Hooke was notorious for claiming credit for inventions done by other people.  Back in the Clement article we noted that Hooke had tried to take credit for the recoil escapement that, as far as we know, he had nothing to do with.  The one thing that we know for certain, is that Hooke discovered a principle, known as Hooke's law, stating that the power of the spring varied in direct proportion to it's displacement.

Robert Hooke, who in 1676 stated,

The power (sic.) of any springy body is in the same proportion with the extension.

announced the birth of elasticity. Hooke's statement expressed mathematically is,

 F = k x u

where F is the applied force (and not the power, as Hooke mistakenly suggested), u is the deformation of the elastic body subjected to the force F, and k is the spring constant (i.e. the ratio of previous two parameters).

(credit: efunda.com)

As early as 1660 Hooke had devised an unsuccessful balance controlled by a straight spring. Hooke was a brilliant man, but he was contentious and disagreeable, and not well liked.  I haven't seen the research alluded to in the wikipedia article, but as far as we know, Huygens was the first to come up with the coiled spring idea, and to get it to work.

(credit for drawings: Clocks and Watches   Alan Smith (drawings by Peter Fitzjohn)  The Connoisseur London 1975)

In it's early form, the balance ran behind the back plate, (often covered by an elaborate balance cock, as in the Tompion watch above the fold) the balance spring was attached, at it's center, to the balance staff, and at the outside to the back plate.  A pair of traveling pins, not shown, followed the curve of the spring, and allowed adjustments for timekeeping.

Huygens had been involved with the idea of an accurate timekeeper accurate enough to find longitude at sea.  There was a lot at stake, as knowing where you were on a ship, determined the power of state and the pelf of commerce. His first attempt was to build a two pendulum clock.  He was the first to note that two pendulums supported from a common support would syncronize with each other, and thus average out errors between them.  The second was a watch who's balance was controlled by a coiled spring.  These ideas were enough advanced that he sent experimental models for sea trials.  Neither was successful, because first, pendulums won't work on a rocking ship, and two, a spring controlled balance varies with temperature not only because of the size of the balance, but by an order of magnitude, the elasticity of the spring.  It would be another hundred years before a timekeeper accurate enough to determine longitude at sea was invented, by Harrison.

In the meantime, the balance spring allowed portable timekeepers to be at least somewhat accurate, and the invention spread as widely and quickly as the pendulum.

I know this issue has been image deficient: The balance spring story is not readily illustrated.  I'll finish up with a picture of a balance spring from a modern Swiss wristwatch:

(image credit: wikipedia)

Next time: A look at early clockmaking in France

Previously:

Monastic alarms and the beginnings of clockmaking
De Dondi's remarkable astrarium
Early tower clocks
Gothic iron clocks
Rennaisance clocks
Early english lantern clocks
Huygens and the pendulum
Fromanteel's English pendulum clocks
Huygens in Paris
Clement and the recoil escapement
Edward East and the golden age
Thomas Tompion
Daniel Quare
Joseph Knibb
Golden age recap