· By Buffy
Isochronism
The property that makes timekeeping possible — the oscillator's period staying constant as amplitude changes — and why perfect isochronism is a myth.
Isochronism (from Greek iso- equal, chronos time) is the property that an oscillator’s period stays the same regardless of how far it swings. It is the whole ballgame: if the period shifts with amplitude, then everything that changes amplitude — winding state, position, lubrication, magnetism — shifts the rate.
The theory
A balance wheel on an ideal spring is isochronous by physics, exactly like a pendulum for small arcs. The problems are all second-order:
- Spring imperfections. A real spiral’s center of gravity drifts as it breathes; end effects make the outer turns behave differently. Breguet overcoils and Phillips terminal curves attack this directly.
- Escapement disturbance. Each impulse is a shove, not a whisper; how much it disturbs the period depends on amplitude. Detached escapements (lever, detent) minimize the contact window; the co-axial minimizes friction within it.
- Air resistance and pivots. Friction is not perfectly proportional to speed; at different amplitudes, different fractions of energy leak through different paths.
Why amplitude is the dial
Amplitude is the visible proxy for isochronism health. A watch that keeps the same rate at 310° and at 220° is isochronous in practice; one that drifts 15 seconds across that range is not, and its daily rate will depend on the hour you last wound it. Timing in positions (see) partially averages these effects, which is why observatory chronometers were adjusted to many positions at many amplitudes.
The mainspring conspiracy
The mainspring’s falling torque slowly lowers amplitude across the power reserve. A non-isochronous watch thus runs at different rates at 10 p.m. and 10 a.m. — the classic “it gains when I wear it, loses in the drawer” complaint. Long power reserves help only by flattening the curve; constant-force devices remove it.
How to judge it
Wristwatch specs almost never quote isochronism directly. The practical proxy: compare the rate fully wound versus 24 hours later, dial up. Single-digit differences mean good isochronism. Chronometer certification (COSC and kin) tests average rates but also requires positional consistency — the closest public evidence you get.