Elinvar is an alloy of nickel and iron with a uniform thermal expansion characteristics. It was used for hairsprings and other springs since its commercialization in 1920, though most hairsprings today use a related alloy known as Nivarox.
Elinvar was discovered by Charles-Edouard Guillaume in 1896. The discovery played a part in his winning the Nobel Prize for Physics in 1920. Guillaume's alloy was commercialized by Fabriques de Spiraux Réunies (SR), a collective of Swiss hairspring manufacturers founded in 1895, and Société des Spiraux Francais, founded in 1919. Similar materials were also used at the time by competing manufacturers, including Invar.
Reinhard Straumann discovered a superior balance spring material in 1931 in collaboration with the German firm Heraeus. This was commercialized by Straumann's company Nivarox SA as a competitor to Elinvar and Invar.
In 1984, Fabriques d'Assortiments Réunies (FAR), SR, and Nivarox were merged to become Nivarox-FAR. Today's balance springs use Nivarox and related alloys based on Straumann's work or modern materials like silicon.