The unit of the thermodynamic temperatures, kelvin, will be redefined after May of 2019, by the new process of fixing the numerical value of the Boltzmann constant. In this respect, the Boltzmann constant, which is a conversion factor between the thermal energy and the thermodynamic temperature, will be assigned as 1.380 649 × 10^-23 J K^-1 after the redefinition procedure. This paper reviews the three experiments which have contributed to the determination of the Boltzmann constant, which are namely: the acoustic gas thermometry, dielectric constant gas thermometry and the noise thermometry. By and large, the physical principles of these experiments are important even after the redefinition takes place, because they are methods used as primary thermometers for the determination of the thermodynamic temperatures. The status of the redefinition and the relation between the thermodynamic temperature and temperature scale is reviewed in this paper.