FIG.III. Index

ISOCHRONOUS. Occurring in equal periods of time. A balance would be truly isochronous if the duration of the oscillations were the same whether the arcs were long or short: if the duration of the arcs were independent of the amplitude of swing. The arcs are affected by - among other factors - The position of the balance (i.e. whether vertical or horizontal) and by the impulses delivered by the escapement. The technique of timing consists in achieving the isochronism of the oscillations of the balance. The chief factors that impair isochronism are: the escapement, the play of the balance spring between the curb pins, faulty poising of the balance and its spring. Pierre Le Roy was the first watchmaker to investigate the supposed isochronal pro- perties of the watch balance and spring in 1760.
John Arnold found by experiment that a helical spring can be made isochronous, or very nearly so, by 'incurving' the terminal coils. See BALANCE SPRING.