Related photographs
No confidently matched photograph of this subject has been found yet. These photographs show a closely related record, and are not examples of the type described on this page.
A plain steel hairspring's elastic modulus falls as temperature rises, which slows the balance's oscillation and makes an uncompensated watch run slow in heat and fast in cold. Early precision watchmaking addressed this mechanically, through the bimetallic compensation balance, whose brass-and-steel rim arms curve inward as temperature rises to offset the softening spring.
Compensation balances work, but they are heavy, delicate and expensive, and they only approximate the ideal correction. The search for a spring that needed no compensation drove the metallurgy. The decisive step was Charles Édouard Guillaume's work on nickel-steel alloys around the turn of the twentieth century — research on invar and elinvar that earned the 1920 Nobel Prize in Physics. Elinvar's elasticity is nearly constant across normal temperatures, so a balance fitted with an elinvar-type spring could dispense with the bimetallic rim.
The interwar years brought the modern alloy family. Nivarox, developed in Germany in the 1930s, is a nickel-iron-chromium-beryllium alloy with a near-zero temperature coefficient of elasticity and the great practical advantage of being non-magnetic and corrosion-resistant. Nivarox-type springs became the industry standard and remain so; 'Nivarox' is a trademark, and many springs are chemically similar alloys from other suppliers.
Material was only half the problem; form was the other. A flat spiral spring 'breathes' asymmetrically as it opens and closes, disturbing the balance's poise in vertical positions. The Breguet overcoil — the spring's outer turn raised and curved inward toward the centre — makes the breathing concentric and improves positional rate, which is why the overcoil remained a mark of high-grade work long after the alloys changed. Terminal-curve geometry was studied mathematically (the Phillips and Grossmann curves) and remains a hand-finishing skill in fine watchmaking.
The most recent chapter is silicon. Silicon hairsprings, made by photolithography from single-crystal wafers, are perfectly anti-magnetic, unaffected by temperature after compensation in the oxide layer, and geometrically exact. They appeared in high-end watches from the early 2000s and are spreading down-market as patents expire — the first genuinely new hairspring material in eighty years.
For identification and dating: a cut bimetallic balance with screws usually means a steel spring and pre-1930s fine work; a smooth monometallic balance means a self-compensating alloy spring; and a blued or purplish spring with a laser-etched look under the loupe may be silicon. The balance and spring together, not either alone, tell you which generation of the technology you are looking at.
Balance springs, regulators and alloys
The integration of the balance spring into timepieces began around 1657, credited to Robert Hooke and Christiaan Huygens, which significantly enhanced the accuracy of portable clocks by allowing them to function as reliable timekeepers. Early methods to regulate the rate of these springs involved the use of various mechanical devices, such as the Tompion regulator, which utilized a semicircular toothed rack, or the Bosley regulator, which introduced a lever pivoted coaxially with the balance. Precision instruments eventually shifted toward free-sprung designs to eliminate the inaccuracies caused by regulator pins, relying instead on timing screws located directly on the balance wheel.
Elinvar, a contraction of the French for invariable elasticity, emerged from the research of Charles Édouard Guillaume, while the trademarked Nivarox was later refined by Reinhard Straumann in 1933.
Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.