Temperature 1 part in 273.16 parts of the difference between absolute zero and the triple point of water; it establishes that one kelvin has precisely the same magnitude as one degree on the Celsius scale; and it establishes the difference between the null points of these scales as being ( = and = ). In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to and the boiling point to . The Rankine scale, still used in fields of chemical engineering in the U.S., is an absolute scale based on the Fahrenheit
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