Keesom, Willem Hendrik

Born June 21, 1876, in Texel; died Mar. 3, 1956, in Leiden. Dutch physicist. Graduated from the University of Amsterdam in 1900 and was a professor at the universities of Utrecht (beginning in 1918) and Leiden (beginning in 1923). He became the director of the cryogenic laboratory at Leiden in 1926. Keesom’s main scientific works were on low-temperature physics (studies of the equation of state of helium, work on thermometry, and so on). In 1926 he produced solid helium. In 1932 he discovered and studied the phase transition in liquid helium at a temperature of 2.19°K, below which it is found in a superfluid state (helium II). In 1935 he found a jump in the thermal conductivity of helium II at a temperature below 2.19°K and normal pressure.