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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.

Charles-Édouard Guillaume sought affordable, stable length standards. His 1920 lecture describes two micrometer microscopes on stone supports sighting marks on a rod immersed in water. Their separation was not fixed: one method alternated the sample with a reference in a second trough at nearly constant temperature. His relative method compared sample and platinum–iridium reference in the same bath, adding the reference's known expansion to the measured difference. Composition and treatment affected the nickel–iron alloys; low expansion held over a limited temperature range.^[ref]

The differential length comparator calculation explains why a calibrated reference is needed even when its expansion is larger than the sample's. Keeping both rods in the same bath targets one source of systematic error; allowing for changes in the instrument baseline targets another. Discovering a low-expansion alloy and demonstrating its dimensional stability are connected experimental tasks.

A nickel–iron alloy with unusually small thermal expansion over a limited temperature range. Composition and treatment matter; its name does not mean every specimen has zero expansion. The Guillaume thermal expansion experiment explains how the effect was measured.^[ref]