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, refined through the nineteenth century with adjustable timing screws around the rim, could reduce temperature error substantially but never eliminated it, and they introduced their own complications, including middle-temperature error, where compensation is correct at two temperatures either side of a central point but imperfect at the centre itself.
The metallurgical alternative was to make the spring itself insensitive to temperature. Charles-Édouard Guillaume's research into nickel-steel alloys produced Invar, prized for its very low thermal expansion, and Elinvar, whose elastic modulus is nearly constant with temperature; the work was recognised with the Nobel Prize in Physics. Elinvar and its refinements (including Elinvar Extra) allowed a monometallic balance to replace the compensation balance in high-grade watches from the early twentieth century onward.
Nivarox, a trademarked family of nickel-iron-based hairspring alloys developed by Swiss industry in the 1930s, became the industry-standard successor, valued for its resistance to magnetism, corrosion and temperature error, and for being non-rusting, unlike plain steel. It remains a generic-sounding but still trademarked name, and 'Nivarox-type' is the appropriate phrase when the exact alloy is not confirmed.
For dating and grading, hairspring material and balance type are useful evidence read together: a bimetallic compensation balance with a blued steel spring points to nineteenth or early twentieth-century high-grade practice, while a monometallic balance with an Elinvar or Nivarox-type spring indicates twentieth-century manufacture, generally from the interwar period onward for Elinvar and from the 1930s–1940s onward for Nivarox-type alloys, though exact adoption dates vary considerably by manufacturer and grade.
Modern silicon and other non-metallic hairsprings, developed from the early twenty-first century onward, are a separate later chapter and outside the scope of this article's historical period; they are not interchangeable with Nivarox-type alloys in servicing an older movement.