Before the invention of GPS, navigating in the middle of the ocean required knowing the exact time at one's port of departure in order to calculate longitude. To meet this vital challenge — the famous “longitude problem” — watchmakers invented the marine chronometer. The saga can be dated precisely: John Harrison completed his H4 in 1759 and had it tried on the Jamaica voyage of 1761-1762; in France, Pierre Le Roy presented a marine watch in 1766 that established the decisive trio — detent escapement, compensation balance, isochronous hairspring — while Ferdinand Berthoud built his marine clocks through the 1760s and 1770s; in England, John Arnold and Thomas Earnshaw settled the industrial form of the detent chronometer and the helical hairspring during the 1780s.
To guarantee exceptional rate stability, they developed the cylindrical hairspring. Unlike the classic flat hairspring, which expands asymmetrically, the cylindrical hairspring unfolds in a perfectly concentric, three-dimensional manner, like a helical spring: the balance's centre of gravity stays on its axis, the condition for near-perfect isochronism of the regulating organ.
This geometry does not work alone: it goes hand in hand with the detent escapement, which interferes with the balance only for a brief instant at each oscillation and thus lets it vibrate freely, and with gimbal suspension, which keeps the chronometer horizontal despite the ship's roll — the three elements form an inseparable system.
It is precisely this helical geometry, developed symmetrically around the balance axis, that gives marine chronometers their isochronism. Abraham-Louis Breguet would obtain the same effect on a flat hairspring in 1795 with his terminal curve: the outermost coil is raised and bent towards the centre so that the spring breathes concentrically again — the solution studied in our Breguet unit.
The history of these extraordinary measuring instruments is a formidable scientific saga that fascinates students and collectors of vintage timepieces alike.
Journey to the heart of historical high precision. Join Décalibre and explore the exceptional mechanisms that have shaped the history of world horology through our 12 course chapters.
Balance-spring geometry and isochronism are treated in the regulation unit of our online watchmaking course, with the reading order set out in how to learn watchmaking online.