Hairspring

The Seiko Museum Ginza describes the arrangement plainly: "The balance wheel comprises a fine 'hairspring' fitted into a metal wheel, with one end of the spring fixed to the wheel and the other fixed to the body of the timepiece." The spring is what makes the balance an oscillator rather than a freely turning wheel: "Like the pendulum, the hairspring is isochronous, so the cycle remains unchanged even when the oscillation angle shifts", and its invention "made it possible to produce timepieces that could remain accurate even on board a violently rocking ship or when carried about".

Its application to portable timekeepers in 1675 is a genuine primary-history item, and a contested one. The Antiquarian Horological Society (a learned society, not an issuing authority) lists under 1675: "Christiaan Huygens invents the balance spring for portable timekeepers at the same time as Robert Hooke claims the same invention." The Seiko Museum Ginza credits Huygens in the same year, then adds: "In the same year, the English clockmaker Thomas Tompion also invented his own balance wheel, based on an idea from his friend Robert Hooke." The year is agreed; the priority is not.

Hairsprings are standardised as components in Switzerland: the Federation of the Swiss Watch Industry's published list of available NIHS norms includes NIHSG 35-10, "Spiraux – Numérotation CGS" (1965/2022), and NIHSG 35-15, "Spiraux normaux – Dimensions de raccordement avec la virole" (1972/2022).

On alloys this entry keeps to a sourced record. The Nobel Foundation's account of the 1920 Nobel Prize in Physics to Charles Edouard Guillaume states that "In 1896 Charles-Edouard Guillaume succeeded in finding an alloy of nickel and steel that registered almost no change in length and volume as a result of temperature changes." That record names invar and scientific instruments, not watch hairsprings, and the composition of any proprietary modern hairspring alloy is not stated here.