A material, invented by Charles Edouard Guillaume , whose "rigidity" (elastic modulus) stays very nearly the same over a wide temperature range. As such, it is useful for clock springs and precision measurement. (Contrast Elinvar -- constant elasticity, as desired for a precision spring, with invar -- constant dimensionality (zero thermal coefficient of expansion), as desired for a length standard.

Guillaume won the Nobel Prize in 1920 for the discovery of Elinvar, invar, and many other nickel steel alloys with characteristics tailored by varying their composition.