What is "invar"

n. An alloy of iron containing 35.5% nickel, and having a very low coefficient of expansion.

Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel– iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes.

It was invented in 1896 by Swiss physicist Charles Édouard Guillaume. He received the Nobel Prize in Physics in 1920 for this discovery, which enabled improvements in scientific instruments.