[3] Guillaume received his early education in Neuchâtel, and obtained a doctoral degree in Physics at ETH Zurich in 1883.

Invar has a near-zero coefficient of thermal expansion, making it useful in constructing precision instruments whose dimensions need to remain constant in spite of varying temperature.

Elinvar has a near-zero thermal coefficient of the modulus of elasticity, making it useful in constructing instruments with springs that need to be unaffected by varying temperature, such as the marine chronometer.

Guillaume is also known for the earliest estimation of the "radiation of the stars” in his 1896 article "La Température de L'Espace" ("The Temperature of Space").

[9] He was one of the first people in history to estimate the temperature of space, as 5–6 K.[9]As the son of a Swiss horologist, Guillaume took an interest in marine chronometers.