Determinations of the Boltzmann constant

Abstract We review measurements of the Boltzmann constant, k , the value of which is soon to be fixed at exactly 1.380 649 × 10 − 23 J⋅K −1 for the future revised Systeme international of units. In addition to a description of the theoretical background and of diverse experimental techniques (acoustic thermometry, Johnson noise thermometry, dielectric constant gas thermometry, and Doppler broadened molecular spectroscopy), the article highlights the decisive role of ab initio calculations of the thermophysical properties of gases, especially helium-4. Perspectives for improvements in thermometry are outlined in the wake of the new definition.