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Charles-Édouard Guillaume investigated iron-nickel alloys whose thermal changes are unusually small. Invar has low thermal expansion over a useful temperature range, making it valuable for dimensional references. Elinvar addresses a related metrological problem through a small temperature dependence of elasticity, useful when an instrument's behavior depends on an elastic element. These materials show why composition and magnetic behavior can matter as much as geometrical precision when building a stable measuring instrument.^[ref]

A physicist.^[ref]

Elinvar names a family of alloys designed to have a small temperature coefficient of elastic stiffness over a useful operating range. This addresses a different problem from Invar: low thermal expansion does not automatically imply temperature-independent Young's modulus.^[ref]

For a simple elastic oscillator, $f\propto \sqrt{k/m}$, so a small stiffness change gives $\Delta f/f\simeq \frac{1}{2}\Delta k/k$ at fixed mass. An alloy with suitably compensated stiffness can reduce thermal drift in a mechanical frequency standard. Its composition, heat treatment and operating range still matter; the name does not mean exact invariance at every temperature.