.avif)
Isochronism describes the ideal behaviour of a watch's oscillator — specifically, the balance wheel and hairspring — whereby the period of each oscillation remains constant regardless of how much energy is driving it. In practice this means the watch should keep the same rate whether the mainspring is fully wound or nearly run down, and whether the balance wheel is swinging through a wide arc or a narrow one.
The concept originates with Christiaan Huygens, who demonstrated in the 17th century that a pendulum swinging through a small arc is approximately isochronous. The same principle was pursued in portable timekeepers through the design of the hairspring. A flat hairspring expands and contracts unevenly under the influence of adjacent coils, causing the effective length of the spring to change slightly with amplitude — which introduces isochronism errors. Abraham-Louis Breguet's overcoil (also called the Breguet overcoil or terminal curve) was designed to address this by shaping the outermost coil of the hairspring so it expands more concentrically, improving isochronism considerably.
Modern approaches to improving isochronism include free-sprung balances, which eliminate the index arm and allow the hairspring to breathe more evenly; silicon hairsprings, which are less susceptible to magnetic interference and thermal variation; and careful regulation of the escapement geometry. Isochronism errors are one of the primary reasons a watch may keep different rates at different states of wind — a phenomenon every owner of an older or poorly regulated movement will recognise.
This is a classic isochronism error. As the mainspring loses tension, the energy delivered to the balance wheel changes, which can alter the amplitude of the swing and therefore the rate. A well-regulated, isochronous movement minimises this variation; cheaper or older movements may show several seconds per day difference between a full and depleted mainspring.
Through careful hairspring shaping (including Breguet overcoils), free-sprung balance designs that remove the index arm, precise adjustment of the escapement geometry, and the use of modern materials like silicon that are less affected by temperature and magnetism. Regulation on a timing machine across multiple positions and power reserve states is the practical check.
Related but distinct. A watch can be consistently inaccurate — running five seconds fast every day — and be perfectly isochronous, because the rate is stable even if it's wrong. Isochronism refers to the consistency of the rate; accuracy refers to how close that rate is to correct time. Both matter, but isochronism is the harder engineering problem.
.avif){kind=small}
.webp)
.webp)
