The temperature at which an ideal gas would exert no pressure. The Kelvin scale of temperatures is defined in terms of the triple point of water, T₃=273.16° (where the solid, liquid, and vapour phases coexist) and absolute zero. Thus, the Kelvin degree is 1/273.16 of the thermodynamic temperature of the triple point. Temperature is measured most simply via the constant-volume ideal-gas thermometer, in which a small amount of gas is introduced (in order to limit the effect of interactions between molecules) and then sealed off. The gas pressure P referenced to its value at the triple point P(T₃) is measured, and temperature T may be inferred from the following equation.