Isochronism is the ideal that a watch’s balance swings at the same rate whether its swing is wide or narrow. It matters because the mainspring loses power as it unwinds. An isochronous balance keeps a steady frequency through that loss, so the watch’s rate stays the same even as the driving force changes.
How do watchmakers achieve isochronism?
In theory, a balance spring is isochronous when it follows Hooke’s law exactly. That means its returning force is proportional to how far the balance has turned. Watchmakers improve isochronism by changing the shape of the spring. The curves at its inner and outer ends matter most. The usual method is the Breguet overcoil. It helps the hairspring breathe in and out more evenly and symmetrically while the balance swings. That even breathing helps the balance hold its rate as the swing grows wider or narrower.
Where it comes from
Galileo Galilei discovered in 1583 that a pendulum keeps equal time. In 1637 he designed an isochronous pendulum clock, but he never finished building it. For the balance wheel, the key step came around 1657. Robert Hooke and Christiaan Huygens added the balance spring, and the balance wheel became accurate. In 1776 John Arnold patented a cylindrical, helical balance spring as part of early work on isochronism. In 1861 M. Phillips published a theory. It showed that a balance spring is isochronous when its centre of gravity lies on the balance wheel’s axis.
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