Thermodynamic temperature in degrees Rankine are converted to kelvins by "dividing" by 1.8 ("T" = "T" ÷ 1.8). Although the kelvin and Celsius scales are defined using absolute zero (0 K) and the triple point of water (273.16 K and 0.01 °C), it is impractical to use this definition at temperatures that are very different from the triple point of water. ITS-90 is then designed to represent the thermodynamic temperature as closely as possible throughout its range. Many different thermometer designs are required to cover the entire range. These include helium vapor pressure thermometers, helium gas thermometers, standard platinum resistance thermometers (known
Absolute zero Δ"H" are nearly the same over a considerable range of temperatures and justifies the approximate empirical Principle of Thomsen and Berthelot, which states that "the equilibrium state to which a system proceeds is the one that evolves the greatest amount of heat", i.e., an actual process is the "most exothermic one". (Callen, pp. 186–187) One model that estimates the properties of an electron gas at absolute zero in metals is the Fermi gas. The electrons, being Fermions, must be in different quantum states, which leads the electrons to get very high typical velocities, even at absolute zero. The maximum energy