French physicist whose exhaustive studies of ferronickel alloys culminated in the discovery of invar (a nickel–steel alloy) and gained him the Nobel Prize for Physics in 1920.

In 1883 Guillaume joined the International Bureau of Weights and Measures, Sèvres, and from 1915 served as its director. His early studies there included exhaustive investigations of the mercury thermometer (thermometer) and of the volume of the litre, which he found to be 1,000.028 cubic centimetres, not 1,000.000 cubic centimetres as had been accepted. From 1890 he focused his attention on alloys and developed Invar and elinvar. Invar's low coefficient of expansion (change in volume caused by change in temperature) and elinvar's low coefficient of elasticity (change in elasticity caused by change in temperature), combined with their low cost, resulted in their widespread use in scientific instruments.