Elinvar is an alloy of nickel, iron and chromium whose elastic modulus barely changes with temperature. That made it a favoured material for the balance springs of mechanical watches and chronometers. The Swiss physicist Charles-Édouard Guillaume, who patented it, received the 1920 Nobel Prize in Physics partly for this alloy.
Why does Elinvar suit a balance spring?
A balance spring works by flexing back and forth. The elastic modulus describes how strongly the metal resists that bending. In Elinvar, this figure barely shifts as the temperature rises or falls. Elinvar springs are usually paired with balance wheels made of nickel silver, also called Maillechort. Even so, an Elinvar spring in a real watch does not keep a perfectly constant elasticity. The name also promises less than it seems. It refers only to the elastic modulus of the metal staying the same, not to the stiffness of the finished spring.
Where it comes from
Charles-Édouard Guillaume, a Swiss physicist born in Fleurier, patented the alloy. He made up the name from the idea of ‘invariable elasticity’. Elinvar is a shortened form of the French phrase with that meaning. The first version was about 52 per cent iron, 36 per cent nickel and 12 per cent chromium. Guillaume gave one of the earliest Elinvar balance springs to the watchmaker Paul Ditisheim.
Not to be confused with
Invar is Guillaume’s other alloy, and the two names are easy to mix up. Invar is valued because it hardly expands when heated. Elinvar is valued for a different quality: it keeps its elasticity steady. One resists changes in size. The other resists changes in springiness. Both are linked to the same physicist, and both deal with the effects of heat, but they solve different problems.
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